Thanks for your response - bought a replacement today from Screwfix for £55.
Problem was that the motor had died on me. Pulled the unit apart without any problem. Next time I will just replace the motor - cost about £6
I have uploaded some of the photos of the innards.
I need to replace this valve on my heating system. The model reference shown on the plate attached to the valve only states "MA1". It is a 3 position valve feeding HW and CH.
Can someone confirm that the model MA1/679-3 offered by Screwfix is a direct replacement?