From the spot on the A3044, if you go to the roundabout and turn left towards Colnbrook, drive a few hundred metres and there is a bit of dirt at the side of the road where you can park, just before the M25 bridge. If you walk onto the bridge you can shoot the a/c as they pass over the M25.
You can see the spot here :-
http://maps.google.co.uk/maps?f=q&hl...=16&iwloc=addr
Also the Queen Mother Reservoir is a brilliant spot if you walk up onto the side of the reservoir as you are so high up, there is very little to get in the way and spoil your shots - you can even get shots with Windsor Castle in the background. The a/c usually start to lower the gear just before they are about to pass over the reservoir, so makes for more interesting shots than the boring side on ones from the A3044 field.
Here's the location :-
http://maps.google.co.uk/maps?f=q&hl...=16&iwloc=addr
Giving all my best spots away now !!!.
