Would I be right in thinking that in a correctly spec'd and configured
CH system, the boiler should really be running pretty much continuosly
if the room stat is demanding heat?

Only with a boiler with a modulating burner. A boiler with a fixed output
rate can't possibly be expected to be perfectly matched to the conditions
and outside temperature etc. every day of the heating system. Rather than
modulating down, such a boiler will cycle. However, provided it isn't too
overpowered and the pump is fast enough, the cycling should be reasonably
long in period. Not on for a minute and off for a minute.

Christian.