Thanks Fuzzyturns,
Noise is certainly a concern, but the inline fan I am looking at is 20 decibels quieter than any proprietary extractor under £500, it's main use seems to be illegal hydroponics where the noise factor really matters! plus I can build a sound insulated cabinet around it outside the workshop as easily as building one inside.
I have also considered bringing the air back into the workshop via a filtered vent in the top of the cabinet, reasoning that if I turn the exhaust upwards the hot air would find it's way back into the cabinet, while in summer I could connect to a duct taking the air deep under the decking and away from the shop.
Clearly the answer is to extend the workshop 3 fold, as I have often mentioned to my wife, but she is still doing the hard stare each time! so I don't think I can do that!
An alternative thought process is to just buy the Record Power DX5000, or Heavy Duty Dust and Chip Extractor (38100) and try to build it inside the workshop as quietly as possible.