In case anyone finds this thread... I think I've worked out why this was erroring.
The site, whilst in dev had - for a while - multiple languages. A doc type had a setting to allow multiple variants (cultures). The languages were later reduced to one but I think this setting was stuck.
This has also caused issues with the Content Service not working (looks fine, doesn't throw an error but then the nodes won't load in the back office with "Cannot save a non-current version").
The fix was I fixed the document type (I used usync but assuming you could add a dummy language, modify in the backoffice then remove the second language). uSync was handy as I could search the text files for this:
FROM:
Culture
TO:
Nothing