Just to update what I've learned through other means today, for anyone else who'd like to know (
@Jamie ********
)...
If I had done my research earlier I would have discovered I was able to apply for the B Visa (renewal) in Thailand, if outside of a month from expiry. But I left that too late so I do now have to exit the country and apply elsewhere, at far greater expense unfortunately - twat tax!
Also, the form WP3 does not appear to be an application for a work permit as much as it's an application for acceptance to apply for one! As I already have a valid Work Permit, this form is apparently not necessary as I can just submit a copy of the current permit.
Apart from some financial documents from my accountants, I was able to gather together everything else needed myself and will not need a Thai lawyer.
I also learned that not all lists of B visa required documents are the same. It seems to depend on where you are applying. The list Robert posted above from the Embassy in Myanmar is much shorter than previous lists I had found online but perhaps it's just less detailed.