Money Box reported that the margins mortgage lenders are making on fixed rate mortgages are the highest they have been for at least 20 years.
On average, banks are charging customers 3.22 per cent more than they are paying for two-year funds, according to financial data service Moneyfacts.
It is costing mortgage lenders less to borrow money, but those savings are not always being passed on to customers.
In some cases, fixed-rate mortgages have been getting more expensive. This week Royal Bank of Scotland raised interest rates on its two-year and five-year fixed-rate mortgages.
Money Box decided to find out exactly what goes on behind those fixed-rate mortgage deals.
Paul Lewis spoke to economist Don Smith, in the dealing room of money brokers ICAP, and then to mortgage specialist Ray Boulger.
This item was first broadcast on Radio 4's Money Box on Saturday, 19 September 2009.