We have a 6v charger with a sealed plug. One wire is black the other has a grey/silver stripe. Which is live which is neutral. Don't want to fry a battery or worse hence the question.

What kind of 6v charge is this? It is impossible to know by just what you are giving us.
Make?
Model?
DC output AC input?

Without more information I'd start by ringing the wires out to the metal body/chassis of the charger , the wire that shows continuity with the chassis is most likely the neutral, if both of them do, it is likely toast.
Why is the old plug missing?