Many thanks as always. The problem was, indeed, upload file size. I "thought" that I had it set to 20M (which indeed I had) but that article pointed out various other settings that also needed to be set.
This is on my local machine with WAMP. I know that the remote server is set to 20M I just hope that the other settings are OK. We'll see.
Anyway thanks for the pointer. It may be that I'll have to split the uploads up a bit rather than try an "all at once" approach.