I'm afraid within the guidlines of CAP371 its legal but unethical.
You need some sort of rostering agreement with your company for it to work, don't hold your breath though, we have an agreement with our OPs Director but crewing seem to think otherwise !
The major problem arises when you are rostered for say 2 earlies followed by 4 lates. You take out the missus, girlfriend, boyfriend what ever, late evening day 2, have a beer or five or bottle of wine or 2, you are OK for the 8 or 10 hours bottle to throttle rule; you are not due to report to say 1400 or if you are lucky 1630 local (our company rule is 10 hours bottle to throttle ) but on arrival at home crewing phone , someones gone sick and they now want you to do the earlies. Clearly you are well beyond the 8 /10 hour limit, you politely point this out but now get accussed of "refusing a duty"and a disciplinary is on the way !
Even if you do the sensible thing and then go sick your self , you are still in the deep and sticky coz they then don't believe you.
Cada da catcha 22 ! Not Good.