Dismantle the key fob, and then examine the battery for the number on it.
Places to try then if you want to go round the high street include jewellers, Currys, Dixons etc., and Timpsons (the shoe repairers / key cutters).
Oh and of course motorcycle dealers and indeed car dealers (many car fobs take small batteries too).
Alternatively this cleverly named internet company
http://www.smallbattery.company.org.uk/ have never let me down. You'll need that number though.
Best of luck
Greg
