Thanks 2.5 miles, but that's a local arrangement particular to Heathrow, and is similar to that we see printed at other airports of significant traffic levels. However, many other places have no such printed speed requirements and it's at these places that I tend to experience the problem of an initial ATC speed given but then never reduced thereafter, leaving the onus on the pilot to reduce at a sensible range....
It bemuses me why anyone would want to cheat at ATC dictated speeds; an extra 10 or 20 knots won't make a damned bit of difference to an arrival time. Perhaps it is a fixation by the pilot of trying to achieve the perfect pre-programmed VNAV CDA without budging the thrust levers out of idle. Anyway with the advent of Mode S can't you interrogate what IAS the pilot has, "spot the liars" and bring them to task?!