Open and deposit £1 into the ING Direct Savings Account. Online account, they stipulate you need a current account that has a cheque book and allows direct debits.
Basically you apply entering the code shown on the page, fill in your details to open the account. Deposit £1 by debit card if it allows you (Some people get the option, some don't), if you don't just send a £1 cheque to the FREEPOST address shown. Once your account is opened 28 days later you'll receive the bonus and you can close the account if you wish.
If you want to use the account as a savings account the interest rate is 2.75% with no penalties for withdrawals, although you get 2.8% with the aa.com. Also please note that ING Direct is not covered by the Financial Services Compensation Scheme as it is covered by the Dutch equivalent.