The first thing I tried was to soak the valve in a solution of "Lime Away" which significantly improved matters but the valve continued to leak slightly. After confirming that the system pressure was actually correct it became an issue of finding a compatable replacement part. Unfortunately this hot water heater pre-dates systems which have a pressure AND temperature blowoff valve. Between that and that the threads on the current hardware are BSP rather than SAE it was off to Oyster to find a replacement pressure valve.
Luckily they still work with the company who made the original water heater and were able to source the correct part for installation.
With this valve installed all is well with the pressure system. Note that this pressure relief valve is 2.5Bar or about 38 PSI. I wonder if they are using a lower pressure switch on the water system as the pressure guage in the system shows about 2.0 - 2.2 Bar when the system is pressurized. This may become an issue when it is time to replace the water system pressure pump.