The replacement Oil Gauge & Sender arrived today so fitted that. It turned out the Gauge was fine, it was the sensor that had failed.
Started the engine and it immediately read 80 psi, which is too high. However, the Voltmeter reads 1.5V too high and the Temperature Gauge reads too high (ECU temperature of 90C shows as 110C on Gauge), so I wasn't totally surprised, Looks like cheap Chinese gauges all over read. These were always just for testing, if I go down the separate gauge route I will get some decent ones.