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Thanks to the extremely well thought through design, if you undo the two silver bands, it slides out from the duct without untaping any joints (hoorah for good design).
Long story short - it was caked solid in moist fluff. Used a 1/2" paint brush to clean about a mug full of s**te out and a balled up old short on a long bit of oval conduit to push through both ducts to clear them out.
Back together, removed exit vent filter and let fan blow remaining crud out vent and now it's as silent as the day it went in.
Given we have no carpets, I was amazed how much s**te was in there - and very grateful I'd not used flexi ducting (except for a foot at the fiddly exit end).