Spot on GB . . . but you could also hook a pair in series and use a 12v charger - if it's easier. Some 6v chargers give a pretty hefty belt, which may or may not be the case with the charger in question, and may or may not be what is needed to get the thing back to life of course.
My own charger, bought in the good 'ole US of A, has a 2A charge-rate option at 12v, which is purrfect for small batts. But at 6v there's only one brutal 10A rate, presumably intended for 6v car-sized batteries.
So for me, connected in series at 12v is better (unless a short sharp shock is what's (maybe!) needed). And the gentle treatment can also be administered by a small battery maintainer-type charger, which I think mostly come for 12v? Mine don't do 6v, anyway.
