[Arran]

Two "D" cells won't last for too long, I was only suggesting that for testing purposes whilst you are trying to get it working. I seem to recall that someone had found a 2 volt rechargeable battery on the other forum for power G.E wet cell portables, that used to have a 2 volt lead-acid cell in the power supply. I don't think it was a NI-Cad but may have been one of those nickel iron phosphate jobs, they either got one from Mouser or Amazon. Another prospect might be an emergency light or burglar alarm battery, those are 6 volt sealed lead-acid batteries, but then you would have to drop four volts of it, maybe by using five diodes in series, or in shunt connection across the terminals, or a power resistor, or in combination.
Regards
Arran