Well, with thanks to everyone on this list I have now successfully (I hope) switched from mbox to Maildir format. Moreover I am now using dovecot-lda to deliver.
I went step by step, and tested it with just one account before opening up the whole system.
I had to rewrite my procmail recipes twice because I first changed them to: | /usr/libexec/dovecot/deliver -d mark -m .MLists.Fedora/
but after testing with the Maildir format up and running I found that they had to be: | /usr/libexec/dovecot/deliver -d mark -m MLists.Fedora
I will now leave it running for a couple of days. I will keep an eye on the logs, but I hope that I have now seen the last of the "Error: Next message unexpectedly corrupted ..." messages!
Thanks again for all the help. Much appreciated!
Mark