Shared mailboxes not shown or accessible

[Tim ODriscoll]

Dear All,

I'm trying (and failing) to implement a shared mailbox folder. My goal  
is to share the inbox of a user 'invoices' to a test user 'rubbish'.  
My ACLs appear to be correct:
# doveadm acl get -u invoices inbox
ID                 Global Rights
user=rubbish              create delete expunge insert lookup post  
read write write-deleted write-seen

The shared-dict is being generated:
# cat /var/lib/dovecot/db/shared-mailboxes
shared/shared-boxes/user/rubbish/invoices
1

Yet when I try to list the mailboxes, the shared namespace has no children:
. login rubbish password
. OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE  
IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS  
THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT  
CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC  
ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS SPECIAL-USE  
BINARY MOVE QUOTA ACL RIGHTS=texk] Logged in
. list "" "*"
* LIST (\HasNoChildren \Trash) "/" Trash
* LIST (\HasNoChildren \Drafts) "/" Drafts
* LIST (\HasNoChildren \Sent) "/" Sent
* LIST (\Noselect \HasNoChildren) "/" shared
* LIST (\HasNoChildren) "/" INBOX
. OK List completed.

I've gone through all the documentation I can find but still can't  
sort this out. Any help would be greatly appreciated.

Tim
My dovecot config:
# 2.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 3.13.0-32-generic x86_64 Ubuntu 14.04 LTS xfs
auth_mechanisms = plain login
auth_username_format = %Ln
default_vsz_limit = 384 M
disable_plaintext_auth = no
listen = *
mail_access_groups = mail
mail_location =  
maildir:/home/email/LHNET/%Lu/Maildir:INBOX=/home/email/LHNET/%Lu/Maildir
mail_plugins = acl
namespace {
    list = yes
    location =  
maildir:/home/email/LHNET/%%Lu/Maildir:INDEXPVT=/home/email/LHNET/%Lu/Maildir/shared/%%Lu:CONTROL=/home/email/LHNET/%Lu/Maildir/shared/%%Lu
    prefix = shared/%%u/
    separator = /
    subscriptions = no
    type = shared
}
namespace inbox {
    inbox = yes
    location =
    mailbox Drafts {
      auto = subscribe
      special_use = \Drafts
    }
    mailbox Junk {
      special_use = \Junk
    }
    mailbox Sent {
      auto = subscribe
      special_use = \Sent
    }
    mailbox "Sent Messages" {
      special_use = \Sent
    }
    mailbox Trash {
      auto = subscribe
      special_use = \Trash
    }
    prefix =
    separator = /
    type = private
}
passdb {
    args = /etc/dovecot/dovecot-ldap.conf.ext
    driver = ldap
}
plugin {
    acl = vfile:/etc/dovecot/global-acls:cache_secs=300
    acl_shared_dict = file:/var/lib/dovecot/db/shared-mailboxes
    quota = maildir:User quota
    quota_grace = 10%%
    quota_rule = *:storage=3G
    quota_warning = storage=95%% quota-warning 95 %u
    quota_warning2 = storage=80%% quota-warning 80 %u
    sieve = ~/.dovecot.sieve
    sieve_dir = ~/sieve
}
protocols = " imap"
service auth {
    unix_listener /var/spool/postfix/private/auth {
      group = mail
      mode = 0666
      user = postfix
    }
    unix_listener auth-userdb {
      group = mail
      mode = 0640
      user = vmail
    }
}
service imap-login {
    inet_listener imap {
      address = *
      port = 143
    }
    inet_listener imaps {
      address = *
      port = 993
      ssl = yes
    }
}
service imap {
    process_limit = 1024
    vsz_limit = 384 M
}
ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_key = </etc/ssl/private/dovecot.pem
userdb {
    args = /etc/dovecot/dovecot-ldap.conf.ext
    driver = ldap
}
protocol lda {
    mail_plugins = sieve acl quota
    postmaster_address = postmaster at lambrookschool.co.uk
}
protocol imap {
    mail_plugins = acl acl imap_acl quota imap_quota
}