If you're a taxpayer, I agree with the others that a Cash ISA is probably the best option to use up your annual �3,000 allowance, over two tax years if you will have the money in a lump sum.
If you're going to be able to save �5000 over a longer term by putting aside a certain amount every month, it's worth investigating the Regular Saver options offered by some banks like Barclays and Alliance & Leicester, where you can earn up to 10% interest for regular savings of up to about �250 a month. However, there is a snag to these accounts in that your main current bank account has to be linked to them.
If you're going for the Cash ISA route, check first that there are no penalties for moving the account as the Bank of England interest rate may well increase in November and you may want to move any Cash ISA to a more competitive rival if the one you invest in no longer offers a good rate.