Check with your mortgage provider,most providers allow you to pay off a % of your mortgage each year before they charge a fee.I have just pad 25% off of my mortgage ag although there was a fee of £625 i am saving £432 pounds a month.

If you pay off the 20k and keep your payments the same you could drastically reduce the time left to pay on your mortgage.This is what I have done and it takes 5years off a 15 year term.

Don't be worried about getting lots of advice phone up your lender and get them to do some sums for you.

Good luck