.....a precautionary Mayday because there was something which may not have been quite right and the crew were playing safe"
Huh? Have another look at CAP413, which states the following for UK-licensed crews in UK airspace:
1.2 States of Emergency
1.2.1 The states of emergency are classified as follows:
a) Distress A condition of being threatened by serious and/or imminent danger and of requiring immediate assistance.
b) Urgency A condition concerning the safety of an aircraft or other vehicle, or of some person on board or within sight, but does not require immediate assistance.
1.2.2 The pilot should make the appropriate emergency call as follows:
a) Distress ‘MAYDAY, MAYDAY, MAYDAY’
b) Urgency ‘PAN PAN, PAN PAN, PAN PAN’
By that definition, it seems that the ba 744 crew must have considered themselves to be "threatened by serious and/or imminent danger and of requiring immediate assistance" IF they made a Distress call during their diversion to Manchester.
Hardly a 'something's not quite right' scenario.
There has been much discussion of the rights and wrongs of this event; what we are really interested to know is the real views of the CAA and FAA over this type of event - airline, a/c type and crew members are irrelevant to the core issue.