Hallf a million pounds were splashed out on the property deal that provided new advice centre premises for the DUP in north Antrim, the Belfast Telegraph has learned.

This £500,000 price, which has not previously been made public, is disclosed in documentation filed with the Government's land registration agency.

The purchase — by a company called Sarcon no 250 — was financed through a mortgage that is being paid off through taxpayer-funded Assembly expenses.

The premises at 9-11 Church Street, Ballymena are the location of a showpiece constituency office run by former First Minister Ian Paisley and his politician son Ian Paisley Jnr.

They moved to the address last year, when Sarcon no 250 was headed by north Antrim developer Seymour Sweeney, a DUP member.

A Bank of Ireland mortgage for the property was secured in July 2007 by Mr Sweeney, as the company’s sole director. The mortgage total involved has not been disclosed.

Legal documentation filed with Land Registry, a Government body, gives £500,000 as the purchase figure

In February this year, Mr Paisley Jnr stressed that Mr Sweeney received no payment for his involvement with Sarcon no 250. The developer stepped down from the firm a year ago.

The DUP Assemblyman also stated: “There is no profit for Sarcon from the rental payments. It goes directly to the bank to furnish the mortgage.”

Mr Paisley Jnr described the Ballymena office as part of the DUP's “long-term commitment” to north Antrim. The rental payments come from the Assembly.

It was disclosed in February that the rent being claimed by the Paisleys for 9-11 Church Street would add up to a joint annual total of £57,000.

The next highest office rental sum claimed by an individual MLA was in the region of £18,000.

Mr Sweeney was replaced as Sarcon no 250's sole director in October 2007 by Mr Paisley Jnr's father-in-law James Currie.

Unlike MPs, MLAs are permitted to claim rent for offices owned by relatives.

In August this year, Mr Currie was replaced as director by DUP councillor Sam Hanna.

According to the latest information filed at the Government-run Companies Registry, Mr Currie remains Sarcon no 250's only shareholder, with two shares in the firm.

Mr Hanna this week declined to comment on the long-term plans for ownership.

He said: “That's a private thing. It's nothing to do with anybody.”

The latest Companies Registry returns state that Sarcon no 250's company secretary is a firm called Sarcon Compliance.

It is linked to Belfast legal firm Carson McDowell.

The law firm declined to respond to questions on its involvement with the Ballymena office, saying: “Carson McDowell is unable to comment on any client business or transactions.”

Mr Paisley Jnr has defended the rental expenses, stating that they had been based on their then market value, as advised by a professional agent.

He further stated that the premises included a meeting place for 150 people, as well as a suite of offices.

Stormont Standards Commissioner Tom Frawley is looking into the office arrangements, as part of an inquiry into whether any of Mr Paisley Jnr's links to Mr Sweeney should have been declared in the Assembly Register of Interests.

The DUP politician denies any wrongdoing and has been cleared of any rule-breaking in a series of previous standards cases.

Mr Sweeney secured the Sarcon mortgage, weeks after Mr Paisley Jnr had lobbied a Government Minister on a land sale plan linked to the businessman.

The MLA also spoke in the Assembly in September 2007 in support of Mr Sweeney's plans for a Giant's Causeway visitor centre.