New sensor solved the problem. The needle moves 2 marks along the gauge and sits there, even on long journeys.
Also, after getting the gauge working it suddenly went right to hot and straight off the gauge! Along with the fuel gauge. On looking, I found a small unit on the back of the fuel gauge that had come loose. It had an earth wire between it and the fuel gauge. On tightening, the earth was secured and all back to normal.
Steve, May 2010