Cheshire Building Society's Platinum Monthly Saver has some catches, but its high interest rate compensates for this. It's suitable for people with lots of savings who already have an easy access savings account.
You get 5% AER until the end of January 2014. The interest rate is variable, although building societies have had a good record recently in not taking advantage of that. My research shows that they don't normally make massive cuts, without good reason, until a deal period ends.