Thanks for the response. Its a fairly basic thermostatic mixer valve...I think. I have striped the thing down, cleaned the valve and enclosure, silicone greased anything that looked like it should move and re-assembled. It seems to be functioning fine now.
If in doubt....take it apart and...
The water temperature from the Heritage exposed valve shower started to fluctuate some weeks ago. At first I thought it was the boiler but that checked out fine. The only way I can get the thing to fluctuate between hot and warm rather than warm and cold is to decrease the water cold water...