What sort of battery? a small one or a car battery? I'm not keen on constantly dragging another car battery around. I'd rather replace the coils - which I'd also rather not do ;)

A small battery will do - a 7.2Ah which is used in most house alarm systems or bikes. You can keep it isolated during cranking with a relay, then activate the relay at a certain RPM threshold (ie 600rpm) using the MS.

Cheers Reverant, just googled a 7.2Ah batter, they're larger than I was expecting, balls.

Maybe I'll have to hunt down some 5v triggered coils and live with a booster in the interim, hooking up a 2nd 1/2 size battery is a bit of a ballache.

Go for a smaller battery, it should be fine.

reckon I could get something to intergrate inside the MS case? or even outside of it, but still under the passenger footwell?

You can try with a 2Ah 12V battery, these are long and slim so it should fit under there.