Just a thought, when one controller issues an instruction and transfers to next frequency, then the next controller gives a further instruction, how does that fit into ICAO's definition? i.e. Is it ATC in general, even different centres et al, or just a particular controller on a frequency??? In the original post the senario is to descend FL130 lvl GWC issued by Latcc AC Hurn Sector 19, any further descent would be by Latcc TC Willo sector on a different frequency, so does the TC Willo instruction override the instruction of a different controller on a different frequency controlling a completely different area of responsibility??
On a side note (sort of fits in, but not quite), and maybe starting up a 'new' thread, when holding is taking place and even EAT's are being given, many a/c ask to reduce speed, but don't want to lose their place in the stack.
1. Reducing speed half way through descent phase may knock a couple of minutes off the stack arrival time, not much more, and rarely affects the landing order. The consideration of a/c type is also used to aid better runway utilisation.
2. By reducing speed, you still have to hit the agreement points at the right level. They are in place for a better flow of traffic, not to get you down for the stack.
3. If you have an EAT, you also have an approach sequence number, this rarely changes so speed reduction is actually favoured to help reduce the number of a/c in the stacks.
Hopefully someone will correct my limited knowledge of the TMA and fill in the spaces.