[Exim] Help with ldap and pam based authentication

(I am not on the list as it is too high a volume, could anyone replying
please cc me, thanks!)

Hi,
I have a little project where I am trying to allow multiple people to
send as specific 'shared emails'. Under my scheme I have pam
authenticating users as usualy for smtp:
plain:
driver = plaintext
public_name = PLAIN
server_condition = ${if pam{$2:${sg{$3}{:}{::}}}{yes}{no}}
server_set_id = $2

ltext:
driver = plaintext
public_name = LOGIN
server_prompts = "Username:: : Password::"
server_condition = ${if pam{$1:${sg{$2}{:}{::}}}{yes}{no}}
server_set_id = $1

As well as a list of authorized mail senders in ldap under the dn of the
sender address. In other words, mail=blah, basedn is a groupOfNames with
member users that can send from it.
dn: mail=blah@blah, basedn
objectclass: top
objectclass: groupofnames
member: bob@realm
member: john@realm

So I tried the following acl after accept hosts in the default exim
configure:

accept  authenticated = *
         senders = ${lookup ldapm
{ldap:///{BASE_DN}?mail?sub?(member=$authenticated_id)}{}}

However, I haven't had much luck with it working. I never get these
exim ldap calls right so I am assuming that it is incorrect. Any help
would be appreciated.

Thanks,
Chris Hamilton