I have done several internet searches to try to identify it to no avail. The shower was getting cooler by the day requiring the temperature tap to be turned up and up. I suspect that the thermostatic valve needs cleaning but I would like to find out what valve it is in case I need spares. I installed it in the 1990s and bought it from an upmarket bathroom store - it wasn't cheap. I think the make started with a T and was something like Thorocks. I managed to unscrew the two chrome tubes on the diverter valves but couldn't undo the middle one. Eventually I wrapped it in double sided tape, a la Blue Peter, and got it to turn but it doesn't seem to unscrew. The nut which holds the valve in appears to be a huge 52 mm across flats so I shall purchase a Land Rover wheel nut box spanner for six quid and hope that will do the job.
If you can help I shall be very pleased. Thank you for your effort even if you can't identify it.