tread, I was fairly sure it was a single, although checking the Islander profile it may be that I just couldn't see the engines - and when I actually heard it last year (directly above me) it did sound remarkably like one of those parachute club flights (slow revving engine). It didn't sound like two engines but I couldn't ascertain that for sure, again because of its altitude. The tailplane wasn't squared, though, from my recollection.
Why would it be so high? Wouldn't that cause one or two ATC headaches? It just mooches over a whole area, takes sharp turns hither and yon at about 0 knots; the area in question can get quite busy when aircraft are coming off the holds and are on the "S" into the Heathrow slope, although that's more like 4000 ft altitude. On the day in question, there were Heathrow outbounds occasionally coming over but again, not very high.
Edit: ah, I read elsewhere that there are three Islanders kitted out to suck up mobile/wifi comms for GCHQ's delectation, flying between FL12 and FL15. Seems a bit desperate, tbh.
Last edited by 3rd_ear; 19th Apr 2015 at 13:39.