Take the bath panel off and put a nrv on the hot pipe
Can I ask how you reach this conclusion from the information provided ?
What was the purpose of turning of the cold feed to the bathroom tub and basin taps to arrive at a problem with the hot water feed on the bath itself ?
Can I not fit a NRV to the hot water supply in the loft instead , rather than removing the tiled bathroom panel ?
It proved that the cold was backing up the hot pipe, yes you could fit it in the loft but the bath would be better if possible.
How can this happen in a mixer tap ?.
Wouldn't the flow of higher pressure cold simply flow into the bath ?.
, you will read on most european cheap stuff that you need to fit check valves