Sorted now, thanks all. On the connection block, on the back of the gauge, the terminals were marked up S, +, and, earth. I wrongly assumed the S was sender and + live. In actual fact the + is the sender and S the live. I had not noticed the two wire into the S. One is power to this gauge and the other is power to the next gauge. I had connected live to the one leg of the switch then the centre leg back to live, hence it worked with the water temp but not with oil as there was no power and the sensor return was connect to the live terminal of the gauge via the switch. swapped over and all working as it should. thanks again all