If interest rates for savers are?

For every pound you overpay your mortgage, you are getting the difference between the mortgage rate (5.39% in your case) and the rate you would earn if "invested" elsewhere. If your investment strategy is to put the money under your mattress (i.e., 0% interest earned), then yes, you would have a 5.39% realized interest on any of that money used to pay down the mortgage. From your question, it looks like you can get 2.5% somewhere (I'd like to know where that is by the way - I have not seen savings rates like that in some time), then any money taken from a 2.5% account and used to pay down a 5.39% account would realize the difference between the 2 rates, or in this case it would be 5.39 - 2.5 = 2.89%.