My Australian common law wife has recently been issued with a Thai work permit. We are not married but have 2 children (birth certificates do exist), I have a UK passport and first entered Thailand on a single entry Non-Imm O visa valid for 3 months. But had to leave for Malaysia in early January and now came back on only a 30 day entry stamp, can i get this extended at the Immigration Office in Phuket? Will they extend my Non-Imm O visa or not as I've already used it?

You have no visa entry - that ended when you departed without a re-entry permit. You have a 30 day visa exempt entry that can not be extended more than 7 days to pack out (and that may require proof of ticket out). The first two sentences have nothing to do with your current situation.

The most you would get is 7 days.

If your wife has a 1 year extension based on her employment, then you can get the same extension based on being her dependent.

A two step process, 2000 Baht to convert your status to non immigrant, then 1,900 Baht for the extension.

If she does not have said extension, then the above posts (7 days only) are correct.

Not sure if the suggestion from Monty will work as our OP is not formally married and Thailand does not recognise de-facto relationships

As said the OP is not married.

Indeed, I read the wife thing wrong and my post was based on the assumption of a legal marriage!

thanks for all your input here, I was never aware of the re-entry permit until yesterday..I will do the obligatory visa run then see what common law's employer can do with my situation after that.