I prefer 'the foggiest fart' gilly. Personally I make too much use of 'blue arsed fly' and 'snowball in hell's chance' and others I wouldn't care to make public 
I was thinking the other day about a silly (harmless) saying I use which would be nonsense to most people but makes perfect sense to me (and those that know me well).
My "covers all eventualities" saying is : "Tiddely Pom" which can be used in so many different ways and has got me out of trouble more times than I can remember.
For example:-
Well, "Tiddely Pom" (meaning that's just the way it is so accept it)
TIDDELY POM? (meaning Bloody hell you are joking aren't you?)
Well that's all Tiddely Pom to me (meaning I haven't got the foggiest fart what you are on about)
.....and you can just go and "Tiddely Pom" yourself for all I care! surely no explanation needed 
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