It's was a combination of two problems. Like everyone has mentioned high pressure is passing over a mixer to exit via a hot outlet. Making showering hazardous and unpredictable. I have tamed the cold mains supply with a prv now and also had to replace control stat on the cylinder as I found this to be clicking off too early. Thanks to everyone for your help on this.
You could also fit a single check NRV to the hot feed of the problem mixer if you can get at it!
Bought some of these as a precaution but don't think they will fit the shower mixer as it has Victorian type riser with shower can at the top (not flexible hose). Only little single check valves as well, so might get blown apart by mains anyway?
That part is no good. I was referring to an NRV on the HOT copper/plastic supply up to the shower valve, if you can get to it!
Like all modern fancy builds the pipes are tombed in either concrete or behind tiling !
