Thanks for the helpful responses - brilliant.
I can proceed with the wiring as above, although one last thought - I am tempted by the idea of a wireless link from the pump module to the boiler.
So the usual room stat devices won't suit. This part of the system is already in place with the room stats and the controller etc. Instead, it really just needs to function as a simple switch. The sender unit could be wired into the pump module with earth and neutral and the switched live (grey wire). When the pump module calls for heat through the grey wire this would trigger the sender unit to signal to the receiver unit which would be wired to the combi in the usual way.
Anyone know if such a sender/receiver device is available?
Mike