The "failing" search you showed earlier was:
Nov 2 11:15:07 pen slapd: conn=8 op=1 SRCH
base="ou=people,dc=ncl,dc=ac,dc=uk" scope=2 deref=3
Nov 2 11:15:07 pen slapd: conn=8 op=1 SRCH attr=cn cn uid
Nov 2 11:15:07 pen slapd: conn=8 op=1 SEARCH RESULT tag=101
err=0 nentries=0 text=
Your slapcat does not have any entries with
and therefore zero results should be returned. As it turns out, zero
results were returned. It looks like the most powerful directory software
on the planet is serving you well.
Uhhhh...so perhaps you want to add this entry? I don't know. It's not an
ACL thing, you've used the rootdn AND you used slapcat (neither of which
was subject to ACLs). loglevel acl can't show anything relevant when
you're using the rootdn. You just don't have the data in there, as proven
by the slapcat. nentries=0, zero results found, absolutely correct.