Ian, with regard to discretion being used to extend flight time to 14 hours, if discretion is used in such a way, this will generally result in the crew member not being used for their following days roster.
According to the IAA, all crew must have an amount of rest equivalent to their duty period, 10 hours being the minimum amount of rest following duty. For a 4 sector day you can generally add 3 hours of duty( assuming there are no delays on the ground). So say a captain were to use discretion to extend flight time to 13 hours, this would mean all crew members would require a minimum of 16 hours rest. Our usual method of dealing with this would be to put all crew members on standby from when they are "back in hours" and hence would, in this situation, only be called if a new crew were needed top operate later in the day or if any crew members offloaded after 2 sectors.
No crew members can be rostered a 12 hour+ shift but for a small number of routes it would only take a delay of 20 mins before discretion may need to be used.