UK APS course teaches that if a pilot reports vacating a level following an instruction to do so, then that level can be allocated to another aircraft immediately; otherwise the controller must wait until the Mode C readout indicates a change of level of 400 feet and continuing in the desired direction before clearing another aircraft to the previously occupied level (obviously common sense dictates that this does not apply to opposing climbing/descending flights!)
UK CAP 493 MATS Part I , Section 1, Chapter 3 Page 3 states:
"
5.4 Changing Levels
5.4.2 An aircraft may be instructed to climb or descend to a level previously occupied by another aircraft provided that:..........
c) either:
i) the controller observes that the vacating aircraft has left the level; or
ii) the pilot has reported vacating the level."
but it also states in Section 1 Chapter 5 Page 10:
"9.3 Level Assessment using Mode C
9.3.1 The following criteria apply when assessing the vertical position of a Mode C transponding aircraft:.........
b) An aircraft which is known to have been instructed to climb or descend may be considered to have left a level when the Mode C readout indicates a change of 400 feet or more from that level and is continuing in the anticipated direction";
Which of the above options is likely to get the job done more quickly??!