Thank you Miki - much appreciated. Cost is important but do want to maintain good condition of battery.It's not really my place to 'recommend' but you do need a modern 'smart charger'.
The CTEK MXS10 fits that profile. there are other makes out there.
Ring, Clarke, Sealey and this one from Draper which is probably the one I would choose as spending over £100 on a battery charger seems a bit OTT
6/12V Smart Charger and Battery Maintainer, 10A
Ensure optimal battery performance with the Draper 6/12V Smart Charger and Battery Maintainer, a reliable 10A solution for charging and maintenance.www.drapertools.com
Simon