Just a quick update on this one.
Checked power going to the main power supplies in the engine ECU box which are fed from the main fused links behind the battery and all good.
Tried connecting jump lead to to earth points and known good earth and to the engine and no change.
I’ll post some more info later but checked power supply to fuse 53 in the passenger fuse box which is the main feed to the ignition switch and that was ok and then tested fuse 5 that is one of the ignition switch feeds out and the voltage was less than 2 volts. I’ll post up the wiring diagram and other fuses that are fed by the ignition switch to test as there are multiple feeds coming out of the ignition.
Allcomms wouldn’t connect most of the main systems so it was looking like power supply.
Conclusion is OP got stuck in to accessing the ignition switch and when moving wires to it the dash lights came back on it also cranked and started. With a bit more persuasion OP says all ok and power stable again so credit due for getting stuck in to what is not a straight forward job of accessing the ignition.
Thanks for all the input from the forum.

Well done chaps. The L322 has a known Ignition Switch issue....

Credit to all who get stuck in!

I think that because this started as a temperature sender issue it was throwing us off the scent a bit but we kept going back to this being something simple , the problem being there are a lot of simple things to check. The problem was evolving quite quickly from temp sender and engine died, temp gauge ok and cranking, half cranking with dash lights , no crank and then no dash lights or column movement with ignition. Allcomms had also stopped connecting to most of the systems so we were flying without a compass. The wiring diagram is a bit tricky to read but once we could see what fuses were fed from the fusible links and what went in and came out the other side of the ignition switch, the sub 2 volts reading was a fairly good combat indicator.

This is part of the starting and charging wiring diagram, there are quite a few fuses in the passenger compartment fuse box that are fed directly from the ignition switch but possibly on separate feeds so one fuse may be good and others bad.

Any large red wires in the engine ECU box should be permanent live direct from the fusible links by the battery. There are fusible links behind the passenger compartment fuse box that i don't think you can access without removing the fuse box, glove box etc which is tedious so i would try and test what they feed.

I didn't get around to testing these but i would assume very low or zero volts the same as fuse 5.
Fuse 34 and fuse 2 should be live with the ignition

The transmission circuit
Fuse 53 and fuse 54 should be permanent live
auxiliary position on ignition - fuse 37 (transfer box) should be live
when ignition on - fuse 5 , 33 and 35(HDC) should be live.

Instruments panel
Fuse 27 (BCU), 46(Instrument panel) and 53 should be permanent live
auxiliary position on key - fuse 45 should be be live
ignition on - fuse 1 should be live and provides two feeds to the instrument panel.
The clock illumination lights should come on with key in auxiliary position and ignition on when the sidelights are on.

I have added some arrows onto the diagram, that is how i interpreted it anyway.