Changed the PRV but that didn't help and after watching it for a bit it was clear that the pressure was rising too high.
Went to check the expansion vessel and it looks like it can be fitted two ways around. Needless to say, it was fitted in the way that made getting a pump on it an absolute pain in the arse.
Very hard to get any sort of pump adaptor on to the valve with it pointing toward the hanging bracket.
With the tank flipped the other way around it'll probably point towards the other bracket (not visible) but will be further from the wall so easier to access.
These things are sent to try us...