Brilliant, thanks Long. I thought it was the right way originally - and I am already using MB Term Meta - but then I had a wobble because when it came to adding the custom fields in code I was trying to do it exactly the same as with a CPT. I'd simply forgotten to look at the MB Term Meta documentation again.
Obviously this is my fault, but if I have one issue with working with MetaBox it's the fact so much is split over so many different plugins and I find it hard to remember to look in the right place to find something when, to my mind, it's all part of the same system but documentation-wise it's in different places.
Thanks so much, I thought I'd have to redo a lot of work - but now I just need to make that " 'taxonomies' => 'category', " change to my code basically!
Best wishes
John