Also
@Kwill it's worth knowing that while most fixed rate mortgages come with a penalty for large overpayments (usually repaying up to 10% extra principal per year is allowed for free, but more than that comes with a charge), those penalties end when the fixed rate period ends.
So instead of overpaying every month for two years, you could just keep the money you would have used for the overpayments in a savings account, then when your fixed period is up, use it to reduce your principal before getting your next fixed period (or not, as you prefer).
Depending on the amounts and rates involved you may not be able to beat your mortgage rate in a savings account – so there might not ultimately be much in it, especially if you aren't eating a higher rate for the privilege of free overpayments. But I thought it was worth pointing out. It's an underutilised option because for most people, if they don't overpay it into the mortgage they'd just spend it.
It's also worth remembering that there may be fees associated with getting a new mortgage deal when this one is up, which is another potential advantage to a longer fixed rate period. (Not all of them have fees, but the ones that don't tend to have higher interest rates, so it's just another number crunching exercise when you get there as to which deal is best).
Ultimately I like to remind myself that it's okay to take a stab at doing a Reasonably Sensible And Advantageous Thing and not stress too much that it might not be the Optimum Thing (that requires a crystal ball). You're already ahead of most of the population here.
