<<whereabouts does the 06.00 deadline apply ? Is it touchdown ? At some 20 nm track here we're about 7 minutes from touchdown which would make 05.53 a key time for someone who needs to just miss the curfew. Or are things not that tight ?>>
Things are very tight as the airport authority penalises any airline breaking the rules. The deadline applies at touchdown. However, because of some quirk with the way time is measured by ATC and the airport authority, the earliest time for landing is 0601. The actual time the first lands is dependent on the skill of the radar controllers who have to take into account the wind and other factors to determine the speed of the a/c, etc. They don't often get it wrong!
Regarding height over Isleworth.. you appear to be just over 4 miles out so the height would be just under 1300 feet.... it's actually 318 feet per nautical mile so you can calculate it exactly!