aj76

OK so no nearer to the cause but have the car running again

The previous owner had fitted a battery isolator so something in my mind made me try using that to turn the power off, replace the fuse, put key and turn to position 2 then restore power. Fuse doesn't blow, usual immobiliser beeps, press unlock on fob to disarm and turn for ignition and the car starts instantly
¯\_(ツ)_/¯

Drives home flawlessly. I am developing a love/hate relationship with this car!