Thats right, it only trips when the CH motorised valve is closed. I
fully opened the by-pass valve, with the CH circuit off, I noticed
that the water wasn't passing through the by-pass valve, massive
temperature difference on either side of the valve.

Sounds knackered, just like most gate valves ten minutes after installation.
When you replace it, use an "Automatic Bypass Valve" instead of a gate
valve. This stays closed when the zone valves are open, but when they close,
it detects the pressure differential from the pump and opens fully. This
way, you get an effective bypass and no short circuit when the system is
actually working.

Christian.