Sorted!!!!
Turned out it was the wiring from the sender to the gauge. In the end I just disconnected the original wiring from the gauge and fitted a new one direct to the sender. Happy days... Reads like it should with a PRC 6663 sender Yippee at last. Anybody want a collection of naff senders that don't match the original gauge. I confirmed the temperature with a IR thermometer and a strap on probe. Probe read about 60deg C on the outside of the top hose and the IR read 81 deg C at the thermostat elbow, gauge read where it should in the middle of H and C. I also checked the viscous coupling using a tacho and it is shot. Reads about half RPM even when bi-metallic strip heated with a hot air gun to 90deg C. So next on the list...... but I do have an electric fan as backup. Belt and braces. This was somebody else's conversion so god knows where the wiring goes to, but I do now