One of the many grey areas in UK FTLs. The highlighted part of Safety's ops manual would seem to cover the situation, however does the phrase
delay to the reporting time due to a changed schedule
refer to a change of schedule to the rostered flight, or does it refer to a change of roster schedule. I would say that the intent of that phrase is when a crew member receives notification that the flight they were planned to operate has been delayed in excess of 4 hours.
I would say this situation is covered by the definition and calculation of Rest Periods.
Definition = A period of time before starting a flying duty period which is designed to give crew members adequate opportunity to rest
before a flight. (the highlight is mine).
Calculation = The aircraft operator must notify crew members in good time of a flying duty period so that sufficient and uninterrupted pre-flight rest can be obtained.
The minimum rest which must be provided before undertaking a flying duty period shall be:
(a) at least as long as the preceding duty period, or
(b) 12 hours
whichever is the greater.
Duty = Any continous period during which a crew member is required to carry out any task associated with the business of the operator.
Did the aircraft operator notify the crew "in good time"? Depends on what you define as "good time".
Could the crew take sufficient and uninterrupted rest? Again, depends on your definition of sufficient, but it wasn't uninterrupted.
How to resolve the matter? Either an unofficial approach to the CAA to seek clarification on the matter, or submit a CHIRP to get an official response. Either way, this situation highlights just how peppered with holes CAP371 (and all its' many interpretations) really is.