that would depend on whats in it! the good thing about england is we have to measure out alcohol in bars so its 2.5 syns for 25ml of spirits alcohol.so if there is vodka, whiskey and rum then that would be 7.5syns (APPROX)then you need to account for your mixer, orange juice (syns per 100 ml), Cranberry juice (3 per 100ml), maybe try and stick to the cocktail that are topped with soda or coke (ask for diet), remember lemonade will be(more than likely) none diet(annoying). avoid creamy cocktails. or you could felxisyn, just enjoy yourself and then draw a line, or what i normally do is enjoy myself and do Success express for a couple of days to get myself together again.
HTH
xx