I'm in a similar position, with a lump sum due. Luckily we have no debts apart from our mortgage.
I had a chat with our building society and we can make a lump sum payment of 10% of the value of the original loan without incurring penalties. (e.g. if you took out a mortgage for �120,000 you should be able to pay a lump sum of �12,000).
As the interest on our mortgage is added daily, it makes sense to do this. We are paying a lump sum soon, then as soon as the new financial year for our mortgage starts, we are paying another lump sum.
We both have the maximum invested in cash ISA's, and as has already been pointed out, interest rates on savings are rubbish right now, so we are going to buy Premium Bonds with the balance.