snip
An interesting analysis - but it doesn't quite take account of the fact
that
a combi boiler EITHER heats the hot water OR the central heating - but not
both at the same time. It achieves this by means of an internal diverter
valve which directs the heated water either through its internal heat
exchanger (for hot water) or through the external circuit (for central
heating).

The symptoms described suggest to me that the diverter valve is not
working
properly - either as a result of a problem with the valve itself or
(perhaps
more likely) as a result of a problem with the logic and sensors which
tell
the valve what to do.

Roger

Aha! As I said, I don't know much about Combis.
I was assuming that it was like conventional boilers which can support
multiple tappings and so could do both CH and H/W (via heat exchanger) at
the same time.

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) or does the diverter flip between the heat
exchanger and the CH circuit all the time, allowing both to appear to be
working simultaneously?