not sure if this is the right forum to put this in, so mods please feel free to move.
MSE has been talking about this and there is loads of informatin in the forum.
How i understand it is when you redeem your mortgage (be it moving to a different lender or paying in full) the mortgage companies charge an exit fee. This exit fee has increased dramatically over the past few years and if it was in your original contract that the charge was less than you were charged when you exited you may be able to claim the difference back.
I have just called the Halifax (08457 273747 - they have a dedicated line) i only changed over in November 2006 (2 year contract) and my original contract stated the charge was £100. Halifax actually charged £175 - so a cheque for £75 is coming to me within 28 days! Worth a phone call!! I didn't even have my role number. Now my brains doing overtime trying to remember who i had mortgages with in previous houses.
Hope this helps someone!