We get this quite a lot at Heathrow, especially with 757s. Of course, because of their particular vortex characteristics, a 757 is often followed by a 747 or 777. When doing Arrivals, I'm often told when the aircraft comes on my frequency that it will be slow inside 4DME, by this time Director will have already put the one behind you on the ILS or just about to establish so it's a bit late to do anything except get the one behind slowed down ASAP (not very easy if its a 777 or 747) or maybe switch one if it's looking exceptionally tight.
I'm sure my radar colleagues will jump in with more deatils, but I'd say that you should tell Intermediate Director (ie. 119.72 or 134.97). Any sooner than that is really not needed. I'd imagine a differential of 10 kts from the norm would be about right.
Gonzo.
[ 04 November 2001: Message edited by: Gonzo ]