This is not a sentence and therefore is not grammatically correct, whereby the necessity of your "period" is irrelevant.
Shouldn't it be: "Failing that: a full stop."? I should have paid attention in English classes at school.
It's all so confusing. I'm sure one thing we were taught at school is that a sentence cannot be just one word. The smallest sentence example was "I ran.". Maybe our teachers didn't know the rules? Wouldn't surprise me.
CofE making way for islam. Thought the Bible stated there is only one God, do not worship false ones, etc. What next, a black* mass in St Paul's? *Satanic, not gospel.