A colleague has one which is not switching the heating at the moment (but
hot water is OK) and I want to see if I can wire the heating circuit onto
the HW relay to get him going until the replacement arrives.

If it just uses the CH On output, then wire the CH side to permanent live.
Control the system with the room thermostat until the new programmer
arrives. Besides a couple of nights with the heating on isn't going to kill
the gas bill. Better still, replace the room thermostat with a programmable
type anyway.

Christian.