The 60 day extension of my non o spouse multi entry visa ran out on 17th June but I'm still okay until July 31 st under the visa amnesty. I was going to fly back to the UK before July 31 but my circumstances have changed. Is it too late to apply for a 1 year marriage extension ? I've had over 400k in a Thai bank account for over 2 months so that part isn't a problem. Cheers.

I suggest you contact your local immigration about applying for the one year extension. If they consider the "automatic extension" under the amnesty as a extension of your entry from your non-o visa you could apply for it.