Now I've got further down the line. I set up a new ou and a smart referral
to the external ou and it's now appearing as I wish:
my domain
ou=people (website users)
ou=staff (us, obviously)
ou=Utilisateurs,their domain
This appears fine in the idm console and I can see all their users. Sadly I
now can't see them through my LDAP admin tool off the server, even logged