Not silly at all Graham, it was reluctant to start and wouldn't idle, I got involved and cleaned up the SU carburettor which fixed it but then noticed the empty filter. The owner is non-technical and can't confirm if it's always pushed air through. I guess that as long as enough petrol is getting to the float chamber in the carb it will run, it doesn't display symptoms of fuel starvation at high revs. Bit hard to tell when an old 2.2 petrol Landy is down on power!
I don't see any air bubbles in the glass bowl of the filter on the pump but it's not easy to from above.
