They funded its purchase in May 2002 with a mortgage on the flat and an extra loan on their family home in west London, then claimed back interest payments for both mortgages on their Parliamentary expenses.

The documents showed that each claimed £1,643.50 a month for what they described on their claim forms as “flat expenses, as agreed”.

The papers also revealed details of a “joint life first death” insurance policy taken out with a mortgage, costing £867.57 a month,  which they expected to be funded by the taxpayer.

Documents submitted to House authorities by the couple in 2002 described it as “compulsory insurance” which they had had to take out because of “age/health etc”.

In 2002, they provided a “schedule of annual costs” to be claimed against their “additional costs allowance”, including mortgage interest, insurance, service charge, ground rent and estimated fuel, Band G council tax and cleaning bills, totalling £41,485.88.

It was not immediately clear from the documents exactly which claims, including the life insurance premiums, were approved but there was evidence that Parliamentary officials had to ask the couple for more detail to support their £1,643.50-a-month each claim.

In 2003, a year after the Keens’ transaction, MPs were banned from using expenses to pay life insurance premiums.

Mrs Keen, 59, has been MP for the west London seat of Brentford and Isleworth since 1997. Husband Alan, 70, has been MP for neighbouring Feltham and Heston since 1992.

The Additional Costs Allowance is designed to help MPs with constituencies away from Westminster who need two homes in order to carry out their duties at Parliament.

There is no suggestion that the Keens broke any rules but correspondence between the Keens and the Commons authorities provides some fascinating insights.

In one letter in May 2004, the couple – in reference to their Additional Costs Allowance claim for the six months to March 31, 2004 – stressed that they had always submitted full details and supporting documents for previous claims.

They accordingly attached statements to back up their latest claim, although they admitted they could not find the council tax demand for the year and had calculated it by taking their monthly payments from bank statements and dividing by two.

“We promise that the claims submitted for 2004-5 will be supported by documents and in a more straightforward manner,” they pledged.

It was not possible to contact the Keens yesterday for comment.

On Saturday, we told that taxpayers  paid nearly £6,000 to decorate Gordon Brown’s kitchen and bedroom.

Full details of spending by the rest of Westminster’s MPs are due to be published in the autumn.