It's Hyperfan Stealth mate, or Hyperfan Silenced. Made by Phresh. Infinitely variable speed controller comes with it, adjustable from 35-100%. I have it on 40% or so and it really is pretty bloody quiet! At 35% there is a quiet "turbine whine" but it's gone at 40%.
200 quid including controller mate. 6 inch. 535 m3 per hour I think. I got one big enough to string two tents together. Just in case!