Saturday’s News Letter revealed that they are making hefty Assembly expenses claims for the premises in Ballymena – and that developer Seymour Sweeney had a role in the purchase of the building.

Now it has emerged that the Stormont allowance they receive is paid to a company whose sole director is Mr Paisley Jnr’s father-in-law, Jim Curry.

And that the property was initially bought only with assistance of Mr Sweeney, whose links to the Paisleys are under intense scrutiny after they lobbied for him to build the Giant’s Causeway visitors’ centre.

Facing fresh criticism, Mr Paisley Jnr last night defended his position, insisting that the office arrangement was legitimate and above-board.

But last night Ulster Unionist leader Sir Reg Empey said these were “very serious developments”.

“If the News Letter’s revelations are true then I believe it warrants a full investigation by the Assembly authorities into Ian Paisley Jnr’s conduct,” he said.

"And there are serious questions here for Ian Paisley Snr to answer, as he was claiming half this amount."

Assembly figures state the Paisleys, together, will pay £62,500 annually in rent for 9-11 Church Street, Ballymena, home to their constituency office.

But Mr Paisley Jnr believes this has been miscalculated and the yearly sum will be closer to £56,000. He wants the tally corrected.

He said that to date they have paid £42,000, since July last year.

This, however, is still much higher than the next recipient, MLA Nigel Dodds, who rents two offices.

Mr Paisley Jn said he had taken the utmost care in ensuring he and his father were not breaching any laws, he noted, and Mr Curry “gets not a penny piece”.

The rent goes straight to the Bank of Ireland with whom a mortgage was secured, in the name of the company Sarcon (No. 250) Limited to buy the property, he said.

Mr Paisley Jnr has accepted he was "the mover and shaker" in the purchase of the property.

It is understood, the Paisleys are not the only MLAs to rent property from a family member. But the details of other people’s affairs are not yet known.

Mr Paisley said: “I have absolutely done this correctly. I did not want to buy the property myself and rent to myself and put my father in an awkward position of renting from me.

“My understanding is that I could have bought it or put it in my wife’s name and she could have charged me. I did not do that.”

On Saturday the News Letter reported that the Land Registry showed the Ballymena office was owned by a company known as Sarcon (No.