Thermostatic showervalve servicing

This photograph is of a thermostatic valve, fitted over 20 years ago. It was supplied through a company called Marflow Engineering, [their Britannia - St James range] who, I understand, had it manufactured by Gummers.

It is in need of servicing in that no hot water is achievable, presumably because the thermostat capsule is no longer functioning. I have been in contact with Marflow but they advise me they can no longer supply spares, nor do they have the capability to undertake any servicing of this unit.

I have noticed, on the Web, a servicing video by a company called Showerdoc whom I have attempted to contact but without response. This video appears to be concerning the Gummers, “Sirrus Mayfair” valve which, in appearance, seems identical to mine.

As I understand you have an involvement with Gummers’ products, is it possible for you to confirm this identification from the detail supplied? If so, is this valve still serviceable, and do you offer such support?

I would be very grateful for any help you can offer before I have to make other arrangements involving a replacement valve.

W. Hockett.