Found Shower Doctor via Google and put a question on the technical forum, which was answered promptly. It seems this is a common fault and the Pilot Valve was recommended as a solution.
The valve was ordered, but was initially out of stock. We were told immediately this was the case and kept up to date as to its ETA and status. It arrived last week slightly earlier than the estimate.
Following the very clear and concise instructions on Shower Doctor website, the Pilot Valve was fitted and the problem was solved. Brilliant, no need to buy a new shower after all!
Excellent service, excellent technical advice, I have no hesitation in highly recommending Shower Doctor and will definitely use again should we have a future problem.