The danger of being sarcastic ...

... is that people take you seriously.

What Sir Jeremy Heywood actually said was:

"We accepted there were unanswered questions including the possibility of a gigantic conspiracy or a small conspiracy. Those were unanswered questions. But we decided, on balance, to let matters rest as they were, decide to stick by Andrew Mitchell, keep him in post and move on."

Now you may consider that the reference to "a gigantic conspiracy" constituted mockery of the possibility.  But the press are more literally minded.  Thus The Guardian reports:

Britain's top civil servant believes Andrew Mitchell, the former chief whip unseated by the "plebgate" row, could have been the victim of a "gigantic conspiracy" involving members of the diplomatic protection group that guards Downing Street.

while The Independent takes a similar line:

Britain’s most senior civil servant was aware that the Government’s former Chief Whip Andrew Mitchell may have been the victim of a “gigantic conspiracy” when he was fighting to save his job - but did not raise his concerns with the police, it emerged today.

Perhaps this will teach Sir Jeremy that in future he should avoid being a smartarse.