Several MPs have challenged a demand for them to repay tens of thousands of pounds in expenses because they rented homes from relatives.

They hope to persuade Sir Thomas Legg, the former civil servant auditing all MPs' claims, to drop his repayment demands because Commons officials actively encouraged them to pay low rents rather than claim mortgage interest of up to £24,000 a year. Over five years, that amounts to £120,000.

Some senior MPs believe that Sir Thomas may soften the payback demands for up to £50,000 – far more than his orders to return money for gardening and cleaning.

One MP said: "This is ludicrous. Some people queried whether they could claim with the [Commons] Fees Office and were told that the rents were lower than the market rate and provided good value for money for the taxpayer. There was no one else to ask."

The inquiry's credibility was called into question yesterday when Sir Thomas issued an "unreserved apology" to the shadow Business Secretary Kenneth Clarke for asking him to repay £4,733 for gardening and cleaning. The figure has now been reduced to £1,345 after Mr Clarke challenged it.

A spokeswoman for the Legg review said: "In a small number of cases where there have been errors we have apologised."

Lord Rennard, the Liberal Democrats' former chief executive, has been cleared of wrongdoing in relation to his expenses following a complaint he claimed £41,000 for staying overnight in London when he owned a house two miles from Westminster.