@evs I think I might have sorted the problem. There were a couple of places causing the error:
In Slot lua around line 45 I changed:
I guess that the original line is Corona syntax?
In SkeletonJson.lua around line 357 I changed:
attachmentName == json.null
to <pre lang=lua>attachmentName == null
as that was the name being given to images that were set to not visible (i.e. removed!).
I think that's all I needed to change to get it working. Can't be 100% sure as I tried several other things along the way while trying to make sense of things.
Thought I'd let you know in case you wanted to tweak your code for anyone who might be downloading it. You'd better check it works if you do in case I've missed anything here.
Quite surprised myself that I found the problem