[Exim] ldap auth and subtree searching

Dear all,
I've been struggling with this for some time. I've been thru all the
documentation and just cant get my head around it, so am in need of an
idiot's guide.

My current config is this (stolen from somewhere on this list;-):

login:
driver = plaintext
public_name = LOGIN
server_prompts = "Username:: : Password::"
server_condition = "${lookup
ldap{user=uid=$1,ou=People,o=sghms,dc=ac.uk pass=$2
ldap://ldap.sghms.ac.uk/ou=People,o=sghms,dc=ac.uk?uid?sub?(uid=$1)}{yes}fail}"
server_set_id = $1

Which works perfectly on the ou=people container, but I want to be able
to authenticate all users in all containers (about 7) below this.

My question is how do I do this? I know I need to so a uid search
somewhere but I really can't fathom where to start.

TIA
Dan