Channel 4 News has obtained evidence of tens of thousands of pounds of spending by the Conservatives during key by-election campaigns which appear not to have been declared.
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Hundreds of pages of receipts obtained by this programme seem to show undeclared expenditure by the party in three crucial parliamentary by-election campaigns in 2014.
The documents appear to reveal a pattern of undisclosed spending and link directly to Conservative HQ and senior figures within the party.
Campaign spending in each by-election is subject to a legal limit of £100,000 to ensure fairness, so contests are not skewed in favour of richer political parties.
If all the receipts had been declared, the party would appear to have flouted spending limits in all three by-elections in Newark, Clacton, and Rochester & Strood during 2014.
All three by-elections were seen as crucial battles to halt the then-growing popularity of UKIP, which was increasingly threatening to steal support from the Tories.
Our investigation obtained copies of receipts for six hotels across the three by-election areas showing stays by large numbers of Conservative campaign workers, including senior party figures.
In all, the hotel bills show 1,401 nights of accommodation, with the total cost of rooms, food, and additional spending amounting to £113,030.63 across the three campaigns.
Of this 1,182 nights with costs of £94,112.19 fell within strict campaigning time limits, known as regulated periods, where spending must be declared by law.
This includes £56,866.75 undeclared hotel bills in Rochester, which would have taken them £53,659.83 over the £100,000 spending limit; £26,786.14 in Clacton, which would take them £10,835.36 over the limit; and £10,459.30 in Newark, which would mean a £6,650.28 overspend.
Under the name "Mr Conservatives"
In each of the three by-election campaigns, we found the same pattern with two hotels booked for Conservative campaign workers -- one upmarket for senior staff, and one budget Premier Inn for more junior campaign members.
Receipts for the all of the six hotels do not appear to have been declared in the material submitted to the Electoral Commission.
The vast majority of these hotel stays - 734 in the regulated periods - were booked under the home address of Marion Little, a senior staffer at Conservative Central Office awarded an OBE in this year's New Year's Honours List for her work during the campaigns.
Some rooms were booked under the name "Mr Conservatives".
Senior Conservative Party figures from the Tories' "Ground War Team", also feature among the bookings including Stephen Gilbert, who was then political secretary to Prime Minister David Cameron, and Simon Glasson, now adviser to Chancellor George Osborne. Stephen Gilbert has since been elevated to the House of Lords following the General election.
Lengthy room stays were booked for specialist senior campaign staff, including the then by-election co-ordinator Richard Piper-Griffiths, and then deputy head of press Richard Holden.
Some of the rooms were paid for from the Conservatives' Premier Inn business account, but Channel 4 News understands that others were paid for in person by Marion Little, in installments.
The scale of the army of campaign workers on the ground - peaking at 42 rooms booked on one night - also call into question staffing costs declared by the party.
If staff were working on the by-election campaigns then the associated costs including all hotel stays and salaries should have been declared.
Any party expenditure for campaigning during the strict time limits, known as the regulated period, should be declared to returning officers by law.
Channel 4 News has also found that at least two campaign centres were also not declared; while the precise costs involved in renting these venues is unknown, it could mean thousands more in campaign spending does not appear on the books.
Rochester & Strood By-election
The Channel 4 News investigation uncovered £72,594.82 in bills at two hotels in Rochester & Strood, of which £56,866.75 fell within the by-election regulated period.
This contest was the last before the General Election, and triggered when sitting MP Mark Reckless defected from the Conservative Party to UKIP.
In response, the Tories threw their weight behind young candidate Kelly Tolhurst.
The largest apparently undeclared receipt obtained by Channel 4 News was in Rochester & Strood, at the four-star Bridgewood Manor hotel in Chatham, Kent.
The receipt includes 19 pages of accommodation costs along with food, drinks, tips, and conference room and equipment hire, totaling £50,228.15.
Five other receipts for the hotel add up to £963.01 - making a total £51,191.16 bill.
Of this, 446 nights' stay and £38,112.83 fell within the regulated period of 24 October 2014 to 20 November 2014, and should have been declared.
At the peak, on the night of the vote, the Conservatives had 25 rooms booked in the hotel, housing members of their "Ground War Team".
It included by-election co-ordinator Richard Piper-Griffiths, and deputy head of press Richard Holden who stayed for 36 nights, including 28 in the regulated period.
Prime Minister David Cameron's then political secretary Stephen Gilbert stayed for one night on 23 October, the day before the regulated period began.
The Conservatives also booked rooms at the Rochester Premier Inn with receipts showing 226 nights in the regulated period, totalling £18,753.92 in accommodation and other costs.
All of the rooms were registered using Marion Little's home address.
Spending returns signed by candidate Kelly Tolhurst, who is now the MP, and her election agent Julian Walden, declare they spent £96,793.08 of the £100,000 campaigning limit.
However, the declaration includes no receipts for hotel stays.
Our investigation also calls into question the amount that was declared for staff costs, which should reflect the salaries of the Conservative workers and was valued at £23,724.
Our findings show a total of 672 hotel nights for workers within the regulated period. If each one represents a day's work, it would work out as a day rate of £35.30, or £4.41 an hour.
Channel 4 News has also found that two Conservative campaign centres in Rochester were not declared, including one in an expensive retail mall.
The centre, Unit 67/68 of the Dockside Outlet Centre, consisted of 3,100 square feet of space in the busy shopping mall on St Mary's Island next to the River Medway.
A quote obtained by Channel 4 News shows the cost including rent, rates, service charge and marketing cost would have been approximately £67,052 per year.
For the 28 days of the regulated period, this would work out as £5,143.71, not including any additional expenses such as utilities and telephone costs.
There is no receipt for the unit in Ms Tolhurst's spending return.
The campaign also used an office on the High Street, Rochester where rent, rates, and phone receipts for the campaigning period would amount to £1,387.91.
Again, no receipt was declared.
It means the full value of undeclared expenditure in Rochester could be even higher.
The Rochester by-election was held on 20 November 2014 with Mark Reckless retaining the seat, which he then lost to Ms Tolhurst in the May 2015 general election.
Channel 4 News has uncovered £26,786.14 of undeclared hotel spending in Clacton.
The by-election was triggered when the sitting Conservative MP Douglas Carswell defected to UKIP becoming the party's first MP amid a political furore.
In retaliation, the Conservative Party waged a huge effort to unseat him on 9 October, with the party machinery called on to promote candidate Giles Watling.
Our investigation found the Tories booked 290 rooms at the luxurious Lifehouse Spa and Hotel, near picturesque Frinton-on-Sea, of which 258 fall in the regulated period from 12 September to 9 October 2014.
At a conservative estimate of £80 a room, it equates to a bill of £20,640.
Again, all of the rooms were booked using Marion Little's home address.
The list of Conservatives staying at the Lifehouse Spa & Hotel include members of the party's "Ground War Team", with George Osborne's special advisor Simon Glasson staying for 28 nights in the regulated period, elections co-ordinator Richard Piper-Griffiths staying for 23 nights, and deputy head of press Richard Holden for 20.
Prime Minister David Cameron's press secretary stayed for the night of the vote, on 9 October.
The party also reserved 71 rooms at the Clacton Premier Inn, of which 68 fall inside the regulated period, with total bills of £6,146.14.
Spending returns signed by candidate Giles Watling and his election agent Victoria Goff declare they spent £84,049.22 of the £100,000 campaigning limit.
However, they make no mention of any hotel receipts, and if the hotel costs were added in the spending would appear to exceed the limits by £10,835.36.
Channel 4 News found an estimated £10,459.30 of undeclared hotel bills in Newark, where suspicions were raised about Tory overspending on the night of the vote, on 5 June 2014.
The election was called after the resignation of Conservative MP Patrick Mercer, and saw the party's replacement candidate Robert Jenrick elected.
But after the count, UKIP leader Nigel Farage questioned whether the Conservative campaign had kept within the strict £100,000 spending limit.
He said: "It is difficult to believe, given the number of paid, professional people from the Conservative Party here... their returns are going to come in below the figure."
Our investigation uncovered undeclared receipts showing the Conservative Party booked 63 rooms at Kelham House Country Manor Hotel, with 59 falling in the regulated period, from 2 May to 5 June 2014.
The total spend at the hotel in the period came to £4,209.30.
Of the 59 rooms booked during the regulated period, 57 were booked under Marion Little's home address, with the hotel housing senior members of the "Ground War Team".
It included Simon Glasson, now Special Adviser to George Osborne, who stayed for 12 nights during the regulated period, the then deputy head of press Richard Holden, who stayed for four nights in the period, and election co-ordinator Richard Piper-Griffiths, for one night.
Prime Minister David Cameron's political secretary Stephen Gilbert stayed for a total of five nights in the regulated period, including four nights in the run up to the vote, and one night on 21 May, where he enjoyed a three course meal including Devon crab brulee and a bottle of Escondido Malbec.
The party also booked 125 nights at the Premier Inn Newark, all in the regulated period, and all under the home address of Marion Little.
At an estimated cost of £50 per room, the bill comes to £6,250.
Again rooms were booked under the names of the Conservatives senior "Ground War Team", including 18 nights for deputy head of press Richard Holden, 18 nights for election coordinator Richard Piper-Griffiths, and three nights for Simon Glasson.
Spending returns signed by Robert Jenrick and his election agent Stuart Wallace declare they spent £96,190.98 of the £100,000 campaigning limit.
Again no hotels were included in the declarations.
The total of £10,459.30 undeclared receipts would take them over the limit.
Channel 4 News has approached all individual candidates and agents for comment, as well as the Conservative Party.
In a statement a party spokesperson said: "All by-election spending has been correctly recorded in accordance with the law."
A spokesman for the Electoral Commission said:
"We have been made aware of the allegations of spending breaches at three parliamentary by-elections. Alleged breaches of the rules around candidate spending at by-elections or parliamentary general elections are matters for the police to investigate under the Representation of the People Act 1983.
"The Electoral Commission only has powers to consider possible breaches of the Political Parties, Elections and Referendums Act at general elections, which is the legislation we regulate. We will be considering whether any of the allegations made tonight also come within the Commission's remit or are strictly matters for the police to investigate.
"In 2013, prior to the last General Election, the Electoral Commission recommended that we should be provided with investigative powers and sanctions for offences relating to candidate spending and donations at specified elections. We have not had responses from either the previous or the current Government to this recommendation."
Investigation by Paul McNamara, Guy Basnett and Ed Fraser.
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