If taking the option of returning to Thailand on a visa exempt entry it is then possible to convert this to a non-O at Immigration, or so I believe, and then to extend after 90 days.
However, it appears that Immigration insist on 'money in the bank' on the day of application and will not accept proof of income - from previous 12 months of international transfers. Is it possible to use an agent to get round this restriction so that I don't need to transfer a large amount of cash to Thailand. I'd much sooner leave it in UK.
The other option, I suppose, would be to get a 'proper' non-O from a consulate abroad as I believe that proof of income is acceptable in that case.