Yep as it's work email, it's the same which ever external email address i use.
It's as if the line
smtpd_recipient_restictions = permit_sasl_authenticated,permit_mynetworks,reject _unauth_destination
Is listening to the mynetworks as in Webmail, yet not for the sasl authenticated users.
Given that when using Roundcube i get
Aug 8 18:20:31 woody postfix/smtp: 7623681847D: to=<firstname.lastname@example.org>, relay=mail.novus.co.uk[18.104.22.168], delay=1, status=sent (250 imss-01.NNL.CO.UK: Message accepted for delivery)
Aug 8 18:20:31 woody postfix/qmgr: 7623681847D: removed
Does that make sense?