http://www.rswww.comBattery, rechargeable, NiMH, memory backup, PCB, 3.6V, 80mAh. Free delivery if you are an account holder.
£2.91
just in case you run out of stock of your premium £6 batteries. :-)
... some other tips while im at it.
Looking for a new fan for your ppc board? Can be hard to find those 30mmx30mm fans but
http://uk.farnell.com have em. They have a minimum order of £20 if your paying by credit card so find some other useful things while you are there. I replaced my fan recently, and put some more heat paste between the ppc and the cooler.
oh.. and
http://uk.farnell.com/productimages/farnell/standard/42246130.jpg£2.90 :-)