The problem we have is the usual one of losing the hot water upstairs
whenever a hot tap is run downstairs.

With this number of bathrooms and outlets, your average instantaneous combi
won't be keeping up if you use multiple outlets at the same time.

You'll need either one of the following:

1. Just lump it and only use one shower at a time and scream at people not
to use the hot taps.
2. Get a storage based solution.
3. Get a combi with more capability.
4. Get a Dr Drivel second combi for one of the bathrooms.

Which solution is optimal depends on many variables.

As to the issue with the downstairs (presumably kitchen) taps starving the
showers, you might consider a local electric heater. They're fine for
handwashing and even doing the dishes, and will prevent the upstairs shower
running cold.

Christian.