Ok changed the sensor this morning, takes about 10 - 15 minutes.
Very very easy from below ( I have a pit).
Do not have to remove any over items.
Go to the far end of the under panel and look up.
You will see the starter motor and starter solenoid.
Just below it is the sensor.
Remove the cable.
You will need a Torx T40 bit. It needs to be a long version not a short bit (the sensor is in the way other wise).
Undo the screw and twist the sensor as well as pulling it.
When it comes out check that the 'O' ring comes out with it.
Refit is reverse.
I have some photos of position and will try to up load them later.
Thanks again for all the comments and help.