If you can tie up your money for 5 years the Yorkshire Building Society will give you 5.3%. Its subsidiary, the Barnsley BS, has a range of one to four year bonds paying 3.1% (1 year) 4.0% (2 years) 4.8% (3 years) and 5.0% (4 years). I think these are the best around at the moment. Bear in mind these are gross rates and should be reduced by 20% if you are a standard rate taxpayer.
National Savings fare slightly better for the shorter term bonds (3.95% for one year and 4.25% for two years).
You can keep an eye on all the latest offers (which are constantly changing) at:
http://www.moneysupermarket.com/