After trawling a bit more I found this forum post
Samba Group...?
It appears that as we didn't use dc=hostname,dc=domain,dc=com but used dc=domain,dc=com in the config_template.xml file we didn't end up seeing the samba group created.
However, when we did used the hostname in the config file we ended up with all kinds of errors, so we removed it then tried again with dc=domain,dc=com and finally have the samba Groups tab available. But... now it seems that all our existing groups and users are missing the object class extensions we need as per this forum post
modify existing user to have Samba attributes
We're going to delete and re-create all the groups now that we can assign appropriate sambaSID and group type then update existing users to get the appropraite objectClass.