Having looked up a pic of the MAF / DEFRA Dornier, i'm fairly certain thats exactly what it was. Having lived overlooking the northern rumway arrival / departure for 8 years, and having worked from home for 3 of them, you do learn to tune out the normal departures / arrivals / motorway / road traffic, and get attuned to something unusual (Go Arounds, helicopters, etc). This was definitely unusual!
I'm crap at giving aircraft heights, but I would have said it was flying in the 2500 -4000ft bracket, circling once towards Heathrow, before heading off south. I'm sure I'd tuned out any departures before this time, there were no departures during this time (probably 5 mins max), but certainly departures afterwards were on 27R, so it would have disrupted anything leaving Heathrow for that period - probably even on 27L.
I would have thought any areal survey work would have been carried out at a higher level - certainly higher than any departing or arriving traffic - and would have required more than 1 pass. I can't honestly believe that it was doing a patrol of the reservoirs - I can't believe that anyone would have thought it would be worth the amount of disruption it would cause, and couldn't be achived more effectively and for less money on foot / van.
So any other guesses?
