Generally the bank will have staff specifically assigned to advising on such matters. I do this myself by over-paying monthly. This reduces the capital balance; however I agreed this and signed up to it in advance. When I enquired I was given a number of options re over-payments and lump-sums and what impact they would have on either the term of the loan or reduced payments (in terms of lump sum). Ring the mortgage department of your bank. Don't make a payment without agreement, as this will only come off the immediate interest and not the capital balance and will not have a long term reduction.