Vaillant VCW242E Overheating and Uncontrollable?

I have a Vaillant VCW242 E Combi Boiler.

Following a freeze up of the Boiler, I can no longer control the temperature of the above boiler. Also the heating stays on 24 x 7 unless I isolate the boiler from the mains.

I have tested the remote controller and room stat and can see the 230-volt switching across terminals 3 and 4 on the boiler terminal strip, as the controller unit is switched between heat and no heat.

However, even if the central heating is switched off using the Black switch on the front of the boiler the heating still keeps operating. (at max temp).

Irrespective of what setting the boiler temperature is set to the boiler will heat up to its Max safe temperature then (I assume) cut out on the over temperature trip. The radiators are scalding hot.

Suspecting that a relay might have fused in the closed circuit position, I have removed the three controller boards in turn to check if any have short circuit relays, but all seem ok and read open circuit at rest. The unit is now re-assembled and is behaving just the same so I doubt it was a loose connection otherwise I would probably re-seated that.

I called in a local plumber who was a very nice chap but I’m afraid he had less of a clue about what the problem is than me.

I would value some opinion from the experts in this forum as to what may be the problem, where I should check next, or what I need to re-check

TIA

Chris.