AVESTUS CAPITAL, formerly known as Quinlan Private Capital, is believed to be on the verge of selling its entire Knightsbridge portfolio in London, which it acquired in 2005 for £530 million.

An informed source said the group had gone “sale agreed” with a buyer from the Middle East. Due diligence is being conducted and Avestus expects to swap contracts in about two weeks.

“It’s a big piece of real estate and a complex transaction,” the source added.

The sale price is said to be “comfortably above” the level paid by Avestus and its investors when they bought the 3.42-acre portfolio five years ago.

It is not clear who the buyer is, but specialist British magazine Property Week recently reported that Saudi Arabian investment group Olayan had teamed up with investment adviser Chelsfield to bid for the site.

Called The Estate, Knightsbridge, it is a freehold island site of about 50,000sq m incorporating the entire prime retail frontage between department stores Harrods and Harvey Nichols in central London.

Avestus has increased the rent roll from the site from just under £20 million a year to more than £26 million annually. On its website, Avestus says the the Knightsbridge portfolio “offers significant opportunities to enhance capital value and income through refurbishment, redevelopment and active asset management”.

Investors in the portfolio are reported to include property developers Bernard McNamara and Paddy McKillen. Avestus adopted its new name at the end of March in a move that severed links with its former founder, financier Derek Quinlan.

Mr Quinlan has relocated to Switzerland and severed his ties with the business in July 2009. He is no longer a shareholder in the investment company.

The deal to acquire the Knightsbridge site in 2005 was financed by three Irish banks, including Anglo Irish Bank.