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The oil pressure on my 64 Mk2 normally runs between 20 and 40 lbs depending on revs and temp. On Saturday, for a period of about 5 mins, whilst driving quite hard up hills and down into the valleys of the peak district it ran with the pressure reducing to zero. It slowly built back up to 20+. The temp gauge went up to 70 degrees (from 50) and the knocking (big ends?) was clearly audible. I drove slowly home (20 miles). On checking when the car was on level ground and cold the oil level on the dip stick was at the lowest 'alowable' point.
Was it a bubble in the system? Is this likely to be fatal? Why were the symptoms temporary?

Your engine must have suffered oil surge while going up & downhill, and may have struggled to draw any oil from the sump if air got into the pick-up pipe. The only way to find out if it's still ok is to try running it.
Check the oil pressure reading with a direct-reading mechanical gauge, as the original (electrical) pressure gauges aren't necessarily accurate.

wiggs,

Have you any update on your oil pressure problem.

Mine (3.4) runs 20-40 psi and temp usually sits on 70 or just above.

How did you get on when you tried running the car??

I can confirm what Pete B says. Use a mechanical gauge to check the true reading, as my car underwent a sudden lack of oil pressure last year when the pressure sender was relatively new (aftermarket type). When I put on an old mechanical type I borrowed off somebody in my local car club the pressure was correct at 40psi at 2000rpm. although it appears to me you have some more problems associated with a drop in oil capacity and I would certainly investigate thoroughly before using the car again.

I topped up the oil and not had a problem since. However, regardless of the type or condition of gauge there was a few minutes where I had no oil pressure. I hope it was just a bubble in the system.

Thanks for your interest and responses.