Ah, the good old 757.
In the UK, as NOD indicates, it is treated as an "upper medium" aircraft (The Uk doesn't have the standard ICAO Heavy, Medium and Light classifications). This creates an interesting situation. Heathrow approach treats it as a medium when following a heavy, thus requiring 5 mile vortex separation. When it is being followed by 737/A320 aircraft 4 miles is used as a votex separation. And when a 757 is being followed by a heavy, it isn't considered to require vortex separation thus 3 miles (or 2.5 depending) is required.