Bit of coincidence here - I read this thread last week, thought nothing more about it and then last night, as I was climbing into bed, what should fly over the house but a pair of very strange sounding helis. I'm out in the sticks just to the north of Grafham Water and get regular overflights of Chinooks, Pumas, Merlins MH53s etc, so am used to the sound of rotors. I missed the first one but was hanging out of the window as the second one came from behind the house and saw it fly away to the south west in the direction of Kimbolton. It was dark so only saw a single red strobe but what struck me as odd was the sound. It was quiet, very quiet by helicopter standards and it was not your normal 'chop chop' sound but more of a whooshing, like a fan. The thing was somewhere around 500 ft I would say and it was going at a fair rate of knots - disappeared from view and out of earshot in a very short time.
Wonder what they were....