09L,
You may recall that the Upper Medium group was introduced to cater primarily for the 757. This following a fair amount of study into the subject.
If you now have some non anecdotal evidence that the spacing should be increased even further, I'd be very pleased to hear it.
In the meantime, I'll just carry on applying the recommended spacing. Perhaps the only saving grace here is that the number of 757's operating into EGLL is gradually reducing.
Thanks for the philosophy lesson
Burp.