This leads me on to another thought - if the combi can only do hot water
or CH does this mean that the house cools down whilst the distaff side
are spending an hour in the shower(s)

Remember that the combi is sufficiently overpowered to get the radiators hot
very quickly when the hot water is not being supplied. Presumably the serial
shower takers will have some sort of three minute changeover between
showers. This would help keep the radiators hot in between.

Christian.