A Slim Shady
You have just re-inforced the point I was making in my first post. If pilots consider the 757 to be a heavy if they are following it, but not if they are in it, it opens up a can of worms where ATC don't know what the separation should be because its all at the discretion of the following pilots or their company.
09L
I'm not suggesting a 757 go 1 minute behind a 777 but there has to be a laid down procedure for this as in the US. We are, like it or not, under commercial pressure to move aircraft at a contracted rate to and from our runways. As burp implies, if every pilot wants another 30 seconds or a minute before departure or you provide and extra half mile on every gap because you think it appropriate, I think you will soon find that targets are not being met and BAA or whoever will want to know why.
TR3
Is it less safe to depart 1 minute behind a 757 than 2?
I don't know, but there are people in offices paid to make these rules on the basis on research and fact. They have said that you can depart 1 minute behind a 757 so 1 minute it is. If you want/need more, tell us early, it may mean you depart before the 757.
In 14 years of controlling I have not yet been notified of any wake vortex incidents from aircraft under my control. Are pilots keeping quiet or just no incidents to report? I do pass type and distance of preceding traffic when separated by the minimum, maybe that helps I don't know.
Until the rules are modified I will continue to use the separations laid down.
Once again, if you want more than the ATC prescribed minimum time before departure, tell us early.