Giles,
We do indeed know what flights have performed a CDA, and which haven't. We are also advised by NATS when ATC have given the pilot incorrect descent information, and remove those from the stats.
The criteria used at LHR are the ones posted above, below 6000ft, +-50ft, 2.5Nm's etc etc.
Oh, and we also monitor track keeping as well, so can you guys please try to fly 09RCPT properly please?
It would make my life so much easier! Its currently below 35%! And if anyone can explain why CPT is so difficult to fly, I'd appreciate it!
Ta