I use Optimate desulphating charger - does the job
It's called Optimate lll. If you google OPTIMATE3.COM you'll find out all about it.
I bought mine for £20 off the guy when I bought the bike but any decent bike shop should have them - I think they are about £40 up here so maybe 50 Euro where you are. Also, check e bay
BTW, interesting choice of user name for an Irishman!!!