I want to drain my central heating so that I replace a faulty mid-position
>motorised valve.
You may not need to drain the system. With some motorised valves the actuator assembly comes off the valve head and can be replaced without undoing the pipework. If it is one of those then you may well find that it is the actuator assembly that is at fault, not the valve itself. Makes it easy to replace the faulty component.
Have you tried taking the head off the motorised valve to see?
PoP