I have pondered for a while the idea of a mains pressure hot water system
but am really concerned about the lack of pressure at the upstairs shower.

Assuming that the mains pressure and flow is good, I'd do it the other way
round. Have the shower (and possibly kitchen tap) off the combi, and run the
bath from the cylinder, possibly using the pump.

Of course, you shouldn't rule out a mains pressure storage system which
gives the best of both worlds. I've just collected such a system this
morning, a 180l DPS Pandora. I'll let everyone know if it works once it is
finally connected up.

I'll obviously need the boiler to be on call for the central heating
and the hot water and these need to be independent.

You wire and plumb it just like you would a conventional heating system. You
will have to discard any boiler built in programmer, as it will only have
one channel. Either lose it, or set it to 24/7.

Decide on either Honeywell 'Y' or 'S' plan. Wire up in the conventional
fashion and lash up the final call for heat to the room thermostat
connection on the boiler. You'll probably need a volt free contact at the
end, so choose a programmer that has volt free contacts, as otherwise,
you'll need a relay, unless the boiler can accept a 230V call for heat.

Christian.