Although traditional grammar says that sentences require a subject and a verb, this isn't true of all sentences.  For instance, I could say "Hello", "Thanks", "OK", "Yes".  

But in the case of your example:

> Now your just being patronising

This *is* a full sentence, but it's misspelt, as you suspected.  The correct spelling is:

> Now you're just being patronising.

where "you're" is a contraction of "you are".  So there's a subject ("you").