I have identified the problem which I mentioned earlier which caused my
incoming mail to not be spooled. All was OK until a particular spam
message using 8-bit encoding. This mail was spooled but with a lot of
'garbage' at the end, but no messages after it were spooled to the nnml
mailboxes. I found 2 of these in the one Incoming file.

It is a bit worrying that a spam mail can stop gnus imported any more
mail from the same /var/spool/mail/user file.

I have tared and gzipped (to stop it breaking anyone else's gnus) the
first offending spam.