Are they nightrider lights?

http://www.wiggle.co.uk/m/cycle/7/NiteRider/

That is almost certainly a 6v charger for the sealed lead acid battery.  Try looking up 6v SLA batts and look for chargers from the same supplier, Maplins may do that kind of thing.

The later model had a NiMh battery, which will be lighter and smaller, easy to tell apart.

TBH, in the long run, you're better off buying a new LED light or torch of some sort.  An older SLA may not take a charge.  I had one of those large yellow rechargeable torches from several years ago, only to find it won't take a charge at all and had to replace the battery.