Hey Steve how are you! Sounds like this gonna be one of those pain in the arse type problems that you hunt and hunt and hunt for ! Hopefully itll sort itself out! Im sure all these guys know alot more then me but for my 2 pence,

id probably start with the oil system flush and oil change! Id empty the oil, put the fresh stuff in along with the flush and run it through for a few (fast miles) , then empty again and replace filter. Then top up fresh oil again! This way your filter or the oil you empty with the flush may hold all the gunk that been hiding in your system.

Saying that does the light on the dash indicate a fault with temperature or pressure... :S

Obviously if the system has been flushed clearing all build up or obstructions inside, along with your sump and pick up pipe as well, that of course just leaves the pressure... maybe a fault with the pump

Then again may just be a faulty sensor
So the hunt begins i guess..... good luck !