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").

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". Some of them might count as interjections, but not all. Then there are phrases like "Once bitten, twice shy", although you could argue that there are words that are "understood", so that the full phrase is "Once you've been bitten, you're twice shy", even though we'd never say that and it actually sounds awkward.

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").