I think there must also be a management issue here.
Say cc leaves home for a 10 hour flight on st to LHR, for a lh flight to the Far East, with only a 5 hour time gap between arriving at LHR and departing as a 'functioning' cc.
Surely that cannot be correct - I know that it is possible to 'nap' in economy, and possibly 'doze' in club, but since cc are important [nay vital according to BASSA] members of the flight team, surely management must be aware of the flight arrangements of individual crew members, and ban them from crewing flights if they have had insufficent 'rest' time.
L337 makes a good point, but when the airline knows of an individual's commuting travel arrangements, surely the airline must ensure that the cc in question arrives in sufficient time for sufficient rest prior to commencement of duty.