Thanks TheTiddles and baldkev, the link about extraction was indeed thought provoking and has steered me in one direction.
Yes, my little extractor is probably not up to the job. It sat outside a 7'x14' shed and my 'new' workshop is a good bit bigger.
This has now raised another question.
I can buy a new extractor and have looked at cyclone machines. But as I have to site the extractor outside the wall (but under cover) anyway, would I be better spending my budget on a more powerful machine to shift more air rather than using a cyclone. I already have a 16A supply there.
I usually try to do all my sanding under the car port anyway to minimise mess, so having a pick-off hose there would be handy.
Meanwhile, I can be planning my duct work...