I assumme that its one of those small gauges that is part of the 3 gauge set up as my 2011 plus 4.
The range of that gauge is 0 to 7Barg, the gauge requires a field resistance range of 10 ohms at 0 Barg to 180 ohms at 7Barg. If you have a suitable potentiometer you can substitute it in place of the pressure transmitter and vary the gauge reading, just connect one end to the spade you pull of the transmitter and one end to engine block (or suitable earth point). You will need a decent multimeter to read resistance.
Regarding the transmitter, ian at Caerbont told me that i required thier model number PTR - 3000 - BN - 10, the 10 indicating that it was 100PSIG transmitter (7 Barg in thier books!). I'm on my second one of these 7 barg transmitters and both have proved inaccurate by approx 1.0 barg at 4 to 5 barg, giving a reading of approx 6 to 6.5 barg which my engine does not kick out.
In furtherance to the above i ordered a transmitter from williams morgan and was sent a 4000 BN 160 which is a 160Psig transmitter. A conversation with Ian again revealed that Morgan always specify this transmitter, ie, the incorrect range for the gauge! However, it appears that the accracy of the 7Barg transmitter is questionable anyway, i tested against a small new bourden tube type gauge which i would assumme was more accurate than the electric transmitter and gauge set up that's on the car. Im not looking for National Physics Lab standards from this kit but would have thought that 5 to 10% accuracy was possible. My latest transmitter would appear to be reading around 25% high.