Pump on hot water circulation does not stop.

Just to give an update.
The problem was with the Honeywell Smartfit Base Unit which controls running of boiler and the pump.
The Micron 40FF boiler has no bearing on the pump.

Purchased the Smartfit Base Unit for 68 pounds and replaced it in 45 minutes.

The lowest quote I had received for parts and labour was 240 pounds. Seems I've saved 172 pounds.

Now I am off to get a hot shower.

Thanks everyone for helping.

I am sorry that you were unable to get a clearcut answer as to what was wrong.

That was simply because it could have been claused by any of several possibilities.

The fact you had an MV replaced and how that was done raised a further possibility of a mistake rather than a failure.

As usual I will mention that any work on an unvented cylinder and its controls and motor valve MUST be done by a qualified person.

The important thing is that your system is now working!

Tony