If you just put the money into a fund then you would have quick access to the money in case your circumstances change, and you need to pay down some of your mortagge, if you have less income in future. The trouble with this approach is that their are not many tax advantages to it, you will pay gains tax as normal on the fund when you sell units.
You could work how much you pay at 41% rate of tax, and once you can claim the full percentage available to your age group invest what you can in a pension plan and get the tax breaks, which are substantial over a number of years. You can claim for the previous year and present years tax. The trouble with this is you are locked into a pension until you are older, so if you were under pressure paying off your martgage later on you can't touch this pension money.
You could as someone previously stated substitute by paying down your mortgage, but use the saving on your current cashflows to pay more to your pension instead, this sounds very sensible.
A lot depends on how much you pay at the top rate, and under how much financial pressure you are under now and will be in the future to make the mortgage payments.