I seem to be missing something from my manual valve, at the moment oil can flow out of it any idea what goes there??

I think its a M10 fine, I am thinking of just putting a bolt in to stop oil flow??.
Rescued attachment HPIM1116.JPG

As the previous owner drilled and tapped it too fit a pressure gauge ?

Yes Jon's spot on,pressure gauge,its handy to no the oil storage pressure in the accusump itself.