Firstly I had to turn on Mail for the parent domain - as far as I can tell this won’t impact on that domain as the DNS is handled externally and points the MX records to Google Apps.
I then had to go to Webmin > Servers > Postfix Mail Server > Virtual Domains and add a record for my mail account, its a shame this isn’t handled automatically, perhaps something to look at by the devs but maybe its just my set-up?
Finally I needed to add the alias domain to the main.cf file, under Webmin > Servers > Postfix Mail Server > Edit config files find the mydestination line add the domain at the end. Again I would expect the system to handle this as it does normal domains/servers - is there a alias type that would handle all alias servers?
Restart Postfix and for me it all now seems to be working!