Is it possible to have the heating circuit off a combi split into 2
zones, one for a ground floor the other for upstairs.
If so, what controls are required, etc,etc?

No problem at all. Wire up as S-Plan. This is compatible with all types of
combi. Some won't handle Y-Plan without external relays, as they often
expect "volt free" contacts on their room thermostat input.

You will need for each zone a 22mm 2 port spring return zone valve and a
programmable room thermostat of your choice.

Simply plumb each zone valve into the circuits for each zone. Obviously, the
feasibility of this will depend on your existing layout. If the upstairs
radiators come of a central trunk, with the downstairs dropping from the
same, you are unlikely to have a suitable layout. The ideal situation is if
near the boiler, the pipework splits into two, with two pipes going up to
the upstairs rads and two pipes going down to the downstairs rad.

Wiring up is simplicity itself. Wire a permanent live (fused at the same
point as the boiler, and preferably using a live out from the boiler, if
available) through the terminals of the room thermostat into the motor
terminals of the appropriate zone valve. Then, take the contact terminals of
all the zone valves, parallel them together and attach to the "room
thermostat" terminals of the boiler.

You may need an automatic bypass valve as well. This will depend on the
model of boiler. They're not expensive.

Christian.