Jonty, as you can tell, this is not a simple question and you really need to get a copy of CAP371 and also the Notices published by the CAA to clarify issues which the original document failed to do.
Basically, CAP371 is a set of rules and guidelines for aircrew and airlines. It is the baseline rules from which each airline must use to develop their own Flight Time Limitations scheme which they then gain approval for from the CAA. In fact, we do not actually operate to CAP371 but our own scheme in the Airline's Ops Manual, however, if it is a 'standard scheme' it is all but the same set of rules. Most airlines will have a few variations however.
It gives guidance on rostering and actual 'flying duty period' allowed under a given set of conditions, the most basic being:
What local time of day do you start your flying duty? How many Sectors are you flying? This determines the number of hours a crew member can be on duty.
To go any further than that I'd be here all day.
Hope the above helps.
[ 02 February 2002: Message edited by: no sig ]</p>