At the present your overpayment should but see later remove £3000 from the debt. This will reduce payments by £3000 x 0.0589 per annum divided by 12 for monthly. However beware since some Mortgage Suppliers see overpayments on Interest Only, as reducing the Interest element by that amount only. It is suggested you ask them whether the first option holds in that the Principal is reduced as Interest Only mortgages sometimes hold the Capital as fixed over the term, then any payment normal, or overpayment is put against Interest. Check this carefully before doing anything.