Just stumbled across this battery charger/optimiser on ebay. Looks like it's reduced in price at the moment too. I know it says for Motorcycle batteries but it's suitable for all 12 Volt batteries. I've used one of these in the past on one of my older cars when it's been stored for the winter and it kept the battery in perfect condition.
12 Volt battery charger / optimiser - free delivery too - Busters ebay shop - £14.99