A Nationwide Flex Direct account pays 5% on saving up to £2,500 for the first year. You do however have to pay in £1,000 each month. However once you have paid it in, a few days later you can transfer it out again.
The best place to look would be one of the comparison sites like money supermarket. Rates will depend on whether you can tie it up for a year or two or more, or want access with being able to give notice. If you want instant access I do not think you will get a a very good rate.
I was with Ing Direct but they changed to Barclays and to stay with them you had to go through a load of palaver with machines and things, I could not understand it
Although you don't have money to pa in each month you could pay some in, draw it out, then pay it back again, and so on. But it may not be worth the hassle. For £5000 even if you can get 3% it's only £150 a year before tax. Depending on your financial position only you can decide how much trouble you want to go to to chase a pound or two extra a week in interest