Have a look on www.moneysavingexpert.com. Martin is great, and he'll tell you where the best place is. I think he's going for an NS&I isa at the moment, if you have �1000 to save.
E.g. if you open an internet ISa with Bradford & Bingley with �1000, you can transfer extra money into it. It pays 5% tax free. You would have to earn 6.25% to get the same interest in an account that had tax taken off (if you pay standard rate). There may be others, that one is instant access I think.