I looked into this charging question recently and it seems you can leave
NiMH batteries on a charger for an indefinite time if the charging current
is limited to 1/10th of the capacity. In your case 120 milliamps. I don't
think voltage matters other than a higher voltage will cause a higher
charging current. For a short period of time you can charge them at a
higher current, at least up to their mAh rating.
I would try to get the highest capacity NiMH batteries that I could find.
This seems to be around 2400 mAh.

On the West side of the Atlantic there are vendors who will rebuild battery
packs for you. Just send in the old one and specify your requirements.
Don't know the costs though. May be uneconomic in many cases.
I would think there would be similar services in the UK.

"Mark Hindley" wrote in message
...

Thanks, I have opened the battery pack, and it is 8 AA in series, as
predicted. If The pack is labelled 600mAh. Presumably they are NiCads.

If I changed to 1200mAh or more NiMH, is the charger going to need
changing? It is nominal 12V supply, and just has a large ceramic
resistor and a diode inside. Do NiMHs need a different charging
current/voltage?

Mark