Thanks. I thought it might be a Drayton as there is a Drayton RB1 relay unit nearby.
I'll make some inquiries along the TA M4 route.
Apparently, the original failed, was replaced, and that failed?? The replacement was sent back to wherever, and didn't turn up. Neither did the original plumber...
Valve does do a 360 without restriction, so no jamming there.
Cheers.