Members of The Portman Building Society shared in a £30 million windfall last year.

The society, which is now the biggest in the South said the payment proved people were better off with mutuals.

In its annual report to members, the society said its assets had increased by eight per cent to £6.9 billion and it had cut its interest margin from 1.45 per cent to 1.28 per cent, reflecting the competitiveness of the industry.

Net receipts were twice the natural market share at £709 million and the society advanced £1.25 billion for new residential mortgages.

Chief executive Robert Sharpe said: "Our performance in the savings market has been nothing short of spectacular. In spite of the competitive market, Portman has continued to grow by providing customers with consistent and attractive products."

He said the society had undertaken extensive customer research prior to investing in new activities such as mobile branches to serve smaller communities, the first of which will be rolled out this year.