It's a problem in the approach phase too - I say "descend now to altitude 3000 ft" and they sit there level ar 4000 ft waiting for the glidepath to come in! The reason I say "descend NOW" is because I want you do go down for traffic reasons. We're playing about at min radar separation and if it starts to get tight we need to get some vertical going quickly, otherwise we get suspended and end up doing lots of paperwork, etc. The fact that you can "see it on TCAS" doesn't avoid this.
Early morning at Heathrow with both runways in use for landing it can be a nightmare - we desperately need those low altitudes for traffic crossing above to the other runway - Jumbo pilots don't like being turned on at 10 miles at 6000 ft - so PLEASE descend when we instruct you to. Thanks..