Do check how your mortgage provider does the calculation. Some do it 'daily', i.e. the sum is deducted from your debt on the day you pay it off, whereas some do it monthly and even worse annually. This means you pay off on 1st Jan a sum (say £10k) but you still pay interest on it for another 12 months! That's a real con, so do check the fine detail.
They should give you a statement each year explaining what you've paid and how much of that was interest and how much capital each month, and how much you have left to go.
Or just phone them, especially if you want to check the exact terms of your overpayment policy.
