It has taken me a while to get round to this, but as I am recovering from an operation, I.ve had some time to get on with it...I will update how I have progressed.
I did as Fred suggested and found that the white wires were the live, in the control panel, and that they feed fuse two (2nd from left) on the control panel and from it, via a green wire, feed the temp gauge, fuel gauge and the ignition light. There was 12 volts going to all THREE gauges. The 1st gauge is engine hours, fed from the 1st fuse on the left and there was a black earth bridging all three gauges. There doesn't appear to be a "cold start" and I don't think Ford engines had one anyways. Thus the workshop manual is useless for the wiring.
I seen this earth was connected to the upper front cab roof but was badly deteriorated so I replaced this earth and , voila, all three gauges sprang to life.
The engine hours and temp gauges are working normally but the fuel gauge is registering as "full". The tank is about 3" from the top but I thought I would check the sender feed anyway.
A green and black wire feeds to the fuel sender, accessed in the battery compartment. I checked that the post on the gauge for this wire is putting out about 4volts carrier current and it was the same at the spade where it connects to the sender. So the continuity seems fine.
However, when I had this wire to the sender disconnected, I noticed that the gauge still read full. If I earth it to the battery, the gauge shows empty so I assume that the sender has a float, which operates a simple potentiometer, which as the fuel level decreases, lowers the float and creates less resistance to the 4volt signal to earth and thus lowers the level on the gauge.
My question is this:- Does the sender unit earth by contact with the chassis of the fuel tank or does it need a separate earth? If someone could look you will see if the sender is connected by one or two wires.
I don't want to remove the top of the fuel tank to see if the float is stuck yet, until I know more about the wiring.
Also, is the sender unit the same as for another vehicle (e.g Landrover) so it would make finding a replacement easier?