Doh...
Swapped the oil pans today and after I got the 750 one onto the Zephyr I noticed a gaping void around the oil filter housing on the Zephyr pan. I made sure all three of the smaller o-rings were in place but forgot the larger one stopping oil from spitting out the side of the oil filter housing. I took the 750 cover back off just to make sure there weren't two o-rings in the engine (there weren't). Then assembled everything and started her up - over double the oil pressure when cold compared to yesterday, will check it warm tomorrow but I think we're good.
First time round I think this was a lack of concentration caused by "I know this engine, just get it together already". This last time I checked and double checked the manual when putting the cases together and sort of got a bit slack at the end. We put the cover on when the engine was upside down on the stand, so I could see the recesses for the small o-rings but not for the filter o-ring. I probably just slapped the oil pan on without having a good look.
I'll put this opne down to "live and learn" - learning to take a bit more time even on the basic stuff. Feeling like a right tw*t but the bike is running again