I have entered Thailand on several different occasions and in my passport I have extended it once for 60 more days to follow my thai wife, I then left Thailand to go back to the UK for two weeks. I will enter Thailand again on a standard 30 day free visa. Can I extend that again to follow wife for 60 days? Or is that stamp only allowed once in my passport?

Yes you can.

Extension is allowed once for every entry.

This all depends on you actually being allowed on the flight from UK.

You need a Visa or a flight leaving Thailand within 30 days.

Yes you can.

Extension is allowed once for every entry.

This all depends on you actually being allowed on the flight from UK.

You need a Visa or a flight leaving Thailand within 30 days.

The requirement to have a flight leaving Thailand within 30 days depends on the airline. Thai immigration does not enforce such a condition. Especially, if you give the impression of being affluent, the airline may not raise the issue. If they do, most airlines have a form they may allow you to sign where you agree to reimburse the airline for any costs they incur should you be refused entry. It is my impression that airlines are becoming stricter, but I have entered visa exempt a handful of times in the last couple of years without an onward ticket. If you are of a nervous disposition, ensure you have one.