car is hot but no temp showing on temp gauge. Nor do cooling fans start (although they run when a/c is switched on). New outlet pipe temp sensor fitted but no change.
car will start and run (rough idle at first) when switched off it turns over but will not start unless left for 4-5 hours.

 

Welcome ianritchie, ...be sure to add this to your favorites.. JagRepair.com - Jaguar Repair Information Resource ...if you do a Site Search for X308 wiring diagram, you'll get the whole cars schematic in "Jaguar XJ 1998"
Have you done a compression check? The unit runs about $15-20.

 

There is only one temperature sensor and that sits in the plastic pipe at the front of the engine. It is cheaper to buy a new pipe with sensor than a sensor on it's own... Especially since the pipe will rot away with age.