The Duke's assurance that all approached are monitored for CDA compliance leads me to wonder if those responsible understand the complexities of a busy TMA? Countless times every day, even every hour, it is necessary for ATC to descend aircraft early or late, thereby preventing CDAs. It's often necessary for us to employ intermediate altitudes on the descent because of other traffic which results in a/c flying level for several miles at 5000 ft. Yesterday we had a guy flying up and down over Heathrow at 6000 ft doing survey work - all departures were stopped off at 5000 ft, ie 1000 ft below their SID level and we had to start descent early on some inbounds and late on others. As has been mentioned elsewhere, during the 6-7am "rush" it is impossible to achieve the REQUIRED landing rate and CDAs so what does the monitoring system make of all this? Are the airlines penalised fofr obeying ATC instructions??