[Exim] Error in rewrite rule

I would appreciate it if someone with a greater understanding of Exim
configuration syntax could take a look at the following rewrite rule and
explain why exim complains that there is a "missing rewrite replacement string".

BTW, it works in the general rewrite section, but when I move it to a
headers_rewrite rule in a transport it fails. (The rule is all on one line,
but the mail client will no doubt have wrapped it.)

headers_rewrite = *@* "${lookup ldap {user=LDAPU pass=LDAPP
LDAPSmail?sub?(&(objectClass=rsppgAccount)(uid=${quote_ldap:$1}))} {$value}
fail}"

The values LDAPU, LDAPP and LDAPS are the ldap server user, password and
URI. They are used in numerous other ldap lookups which work perfectly.

It seems as if the quoting isn't having any effect, is this possible?

--
Nigel Wade, System Administrator, Space Plasma Physics Group,
             University of Leicester, Leicester, LE1 7RH, UK
E-mail :    nmw@???
Phone :     +44 (0)116 2523548, Fax : +44 (0)116 2523555