As I managed to work around the problem by implementing SFTP I doubt it's anything to do with Fail2Ban but I'll try your suggestions and get back to you.
Being Debian we don't have /var/log/secure but we do have /var/log/auth.log
These are entries for a session on Monday 6th when the process worked
Jul 6 09:03:38 vps02 proftpd[2429803]: pam_unix(proftpd:session): session opened for user xxxxxxxxxx-parts(uid=10002) by (uid=0)
Jul 6 09:03:38...