Goldberg, thanks for the reply. I have checked the spindle and it is moving quite freely. With regards to the pricing a couple of examples are:
UK Plumbing - V4073A1039 £56.93, 40003916-003 Replacement Powerhead for V4073A £60.69.
Heating Controls Online - V4073A1039 £49.40, 40003916-003...
Over the past week my central heating system has been playing up so at the weekend it was out with the multimeter to track down the problem. I came to the conclusion that I need to replace the powerhead on my diverter valve. As the powerhead on the V4073A1039 is easily replacable I thought great...