We just overpay what we can each month - sometimes £50, sometimes more (thank you ebay) we can pay off £500 per month without penalty and get the money back out again if we ever need it and in the meantime save lots on interest. We ask for a reduction in the term of the mortgage but you can ask for a reduction in the amount to pay each month (saving that for when we are broke!)
Premium bonds are really not a good idea IMHO if you're actually wanting to make some interest on your savings - you'd almost certainly be much better paying it off your mortgage than putting in premium bonds. Have a look here
I would speak to your financial advisor because what is the best thing to do with it will really depend on what you plan to do with it in the long term. Don't forget that you need to earn around 6% interest (iirc) to take into account the tax you pay on the interest and inflation.
If you want a long term investment then a shares ISA might be worth considering as the stock market is so low at the moment.