Remember the MATS Part 1 is NOT the sole document which ATC have to abide by. We also have the AIP and the CAP 413. Seems
black and white to me from those other documents what to expect. But then I always was a simpleton and liked the easy way out in life
CAP413
Chapter 3 Page 3 Paragraph 2.3.4
Pilots are expected to comply with ATC Instructions as soon as they are issued. However, when a climb/descent is left to the discretion of the pilot, the words 'when ready' shall be used; in these circumstances the pilot will report 'leaving' his present level. Should pilots be instructed to report leaving a level, they should inform ATC that they have left an assigned level only when the aircrafts altimeter indicates that the aircraft has actually departed from that level and is maintaining a positive rate of climb or descent, in accordance with published procedures.
UK AIP
ENR 1-1-3-1 Paragraph 2.2
2.2.1 In order to ensure that controllers can accurately predict flight profiles to maintain standard vertical separation between aircraft, pilots of aircraft commencing climb or descent in accordance with an ATC clearance should inform the controller if they anticipate their rate of climb or descent during the level change will be less than 500 feet per minute, or if at any time during such a climb or descent their vertical speed is, in fact, less than 500 feet per minute.
2.2.2 This requirement applies to both the en route phase of flight and to terminal holding above the Transition Altitude.
Note: This is not a prohibition on the use of rates of climb or descent of less than 500 feet per minute where necessary to comply with other operating procedures.
Do we really need to reinvent the wheel ???