Oil strainer and oil pressure relationship?

And you already said the sensor works. Sorry to ask, but did you put oil in?

 

Damn it I knew I was missing something! I wish so much that that was the answer. Going to order a new switch and stick that in.

 

Turns out to be the external oil filter. Removed the pipes from it and temporarily joined them through and bingo the light goes out. The only explanation I can think of is the suction as the oil drained must have pulled the one way valve in the filter shut so tight it won't open. My next question is what is the correct filter for it. The last time I replaced it I just swapped it like for like, it is a Halfords HOF206. Just wondering if it should be a different type that maybe does not have the valve?