So I don't want to tempt fate, but I think I've fixed it

Well, I've found the problem - need a few bits from Halfords to fix it properly.

There's a bit of a rats nest behind the dash, looks like someone had a fit with a bowl of spaghetti! Among it all is a loose negative wire which was rubbing against the oil light housing, I moved this and the problem was solved
I just need to find out where this wire should be as everything else seems to work...
Also whilst wiggling the wire under the bonnet the connection to the sensor has snapped, need to replaced this for the MOT...

Thanks for all the help and suggestions