If one has a wood fired stove (as opposed to a dedicated boiler) how
does the hot water/CH system work? What happens to the 'spare' heat
when there's too much wood on the fire and no one is having a bath?

The hot water cylinder gets hotter and hotter until it boils, unless you
have an additional radiator for heat dumping. Having a full radiator circuit
is a good idea. It is very likely that if you want heat on the fire, then
heat in the radiators would also be beneficial.

It is best done using a heat bank system, with primary gravity ciculation
for the solid fuel back boiler. You can then add an additional gas/oil
boiler for convenience and the radiators will work off either.

Christian.