Thanks for comments everyone.
The photo was shot with an image stabilised lens, at max telephoto, at 1/60 sec as the light was fading. That probably accounts for some of the blurriness (it was also darned cold!)
The reason I took the photo was that by eye, the separation looked less than usual. I have lived near EGLL for over 16 years and fly in/out several times as year. I have seen how close traffic separation gets in the turn onto final when a couple of streams are being vectored off the stacks and joined onto the localiser. I have seen this from the ground, as a passenger in the cabin and as a guest in the cockpit. I have also done a little formation flying in a Cessna a few years back.
All my experience made me think - oooh, a bit closer than usual, so I took the shot.
I get a slightly different image ratio to TimS (he gets 1.42, I get 1.24 for wing length, 1.21 for nose to tail). But we'd need exact specs on the a/c dimensions to take the analysis much further. I agree that the a/c on the right looks slightly higher, but one aspect that the still shot does not bring out is that they flew as if in formation. I had time to see them, get my camera out of the bag, power up, focus and take the shot. For about 10 secs they maintained relative position.
Then again, they could be on the same airway (perhaps T420?) at different FLs, with a bit of lateral sep.
Anyway, since I have not heard of any airprox report, I'll follow the general consensus that they were indeed separated.
Cheers,