Ian
The follow up to Easyjets fatigue study was they moved from 5/2/5/4 to 5/3/5/4. The study suggested I believe, that it took more than 2 days off to recover sufficiently from 5 earlies (although I would add Ground Staff have to
suffer still). The 900 hours limit is sufficient to protect crews from suffering long term fatigue in theory. Fly to PMI 5 days a week at 0900, back at 1700, have 2 days off and so on. However with the mixture of earlies, lates, nights, only Pilots that work for EZY/FR/BA can tell you.
Ryanair operate a 6/3/6/3 so in theory 2 extra days to EZY Crews, and because they work to Irish FTL limits, can work longer days and more sectors, on average around 1hr more a day and 5 hrs more a week.
In terms of going over the 100 hr limit most airlines can get their crews to that limit, especially in the summer months. Going right to the 900 annual limit is a Esy/FR/BA specialist subject.
With ref to the 12 hr shift limit you need to do some more homework on CAP371 the UK CAA regs and the Irish regs. Its not a simple 12hrs, in the UK it depends on time of report and sectors flown, the more you do the less hours you can work. The funnier the time you report the less you can work etc depending if your body says you should be asleep etc.
Commanders discretion is allowed on the day to recover situations and the maximum is 3 hrs, most crews don't go over 2 hrs as 3hrs the report goes to the CAA.
Last and by no means least, please remember that UK and even Irish rules are restrictive compared to the Spanish, and Eastern Block countries. And last and by no means least, with all due respect, pilots in general don't like coming to work!. When they get to work they are fantastic, god like people whom I have the utmost respect for.