Had considered a couple of those things, but another motor is on its way soon so won't get too into it for now. Oil \"looks\" ok though. I actually now suspect the return oil pump or blockage somewhere, as the level was way down (not registering!) on the dipstick after I heard the hissing, even after letting it settle for a while. I topped it up a bit, and then ran it for a few k's, checked again and found it to be overly full (and no pressure), indicating possibly that the problem had \"fixed\" itself. So all the oil must have been trapped in the cases - hence the pressure??
Funnily enough, back in the 70s, including road racing, street drags and general everyday thrashing, I never even knew there was a filter screen so it was never checked over a period of about 30k miles!! Never even had the motor out - they're a tough breed.....
Thanks for your input.