@Sachakins mentioned Gyro type dust collectors/separators, which prompted me to go back and take a look at the Harvey/Axminster G700. I was quite impressed when I saw the capacity as 1110cfm with 6" ducting ... but this is then contradicted when you scroll down to graph slightly further on, which shows machine maxes out at ~750cfm, rough guess will deliver perhaps up to 600cfm with the sort of set up I expect to have ... which isn't really enough for 6" duct. The bigger Gyro collectors don't have graphs ... but if there's a single message I've taken from the Pentz site it's not to buy an extractor without studying the graph.
Back in the cyclone world, overnight I sent out a few requests to industrial suppliers to quote for material moving fan able to deliver 2000 m3/h at 2500Pa (1150cfm at 10" pressure): this looks like the best route, I am getting helpful replies.