Hi Nigel,
It looks as if there is either wear in the engine plain bearings or in the oil pump,
or a combination of both???
My Fathers old grey Ferguson behaved like your BSA!! 0 psi when the engine was hot
You could try running the engine for a little while, with the timing cover removed when hot
you should be able to see if the oil pump is leaking (a lot??)
This is a messy test!!!!!!
There are a couple of suppliers of either iron or alloy bodied pumps
Another issue can be if either the fibre washer under the front mounting screw is either missing or too thick!!
What I do is make a gasket like an A65 one that extends forward to include the front screw.
have a look at this as well.
http://www.srm-engineering.com/technical/wet-sumping-problemsDont complain too much about 30 degree temps!!!
It looks like winter is on the way here in Ireland!!! after we have had the wettest summer on record!!!!!
Regards
John O R