Head over to Zopa to find out exactly how you can invest, and to check out your options, which range from a low-return scheme which will release your funds whenever you need them, to a scheme with much higher interest but a greater risk of your borrower defaulting.
You have to spend money to make money, they say, so why not invest your excess funds to help people who need it? Peer-to-peer lending is a great way to grow your cash with minimal risk.