Whether to diverify or not is probably a decision you have to make yourself depending on your circumstances, for instance if you were to pay some of the money into a pension then you would receive additional tax relief on these payments and as you are both in your twenties it is a good time to start, generally speaking paying off your mortagage early is a very good idea and you can always reduce or take a break from the lumpsum payments as your mortgage reduces, at the rate you are paying 40K per annum you should see your overall mortgage reduce quite dramatically.