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