> I've been told that a Lithium battery would be a good replacement.
Nope. Totally unsuitable, and they can have a very short life unless you muck about extensively with very low power RAM chips.
> So, I think I might fit a Lithium battery (CR123a type or similar)
Fit an NiMH, they're a drop-in replacement.
> I really don't want to use ni-cad batteries any more, I've seen so much good gear ruined by them.
You can't even get NiCad backup batteries any more. Just get a NiMH, fit it, and forget it for another twenty years.
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Gordonjcp MM0YEQ