[exim] DKIM with exim

I have become interested in using DKIM verification with exim. I built
exim-4.71 and used a version of configure containing...

acl_smtp_dkim = check_dkim

...

dkim_verify_signers = $dkim_signers

...

check_dkim:
warn log_message = DKIM status for $dkim_cur_signer =
$dkim_verify_status ($dkim_verify_reason)
accept

When I tested it with a sample ebay message it didn't seem to do
anything so I captured lots of other messages with DKIM signatures, in
particular one containing this anonymised header:

DKIM-Signature: v=1; a=rsa-sha1; c=simple/simple; d=anonymised.ac.uk;
         s=200903; t=1270808933; bh=w9Xw4DeVUsAPIrNhbwDFKGYLVig=;
         h=From:To:Subject:Date:References:In-Reply-To:MIME-Version;
         b=oBx2b1XZ0kl1IFUY4wgojoP7a7hzaiw7NNTf0XBGG75mPRTOf4Z5NW+AIAz/E4smo
          RISE8HZYRZzm2ZhnNyvumYAGkbAtpFMgeTIsD/ZtHW1NERGHeWrGjeZwPqbgwJvwtR
          AJ9lCX8eJ7CXbwHEJVuJAOiOYLIRL6t/7Q9qt8CA=

However, when I tested the configuration with the message using -bh it
just put out the line

PDKIM <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

When I forced it to do DKIM by using

dkim_verify_signers = $sender_address_domain:$dkim_signers

I got

PDKIM >> Hashed body data, canonicalized >>>>>>>>>>>>>>>>>>>>>>>>>>>>>
PDKIM <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
>>> using ACL "check_dkim"
>>> processing "warn"
>>> warn: condition test succeeded
LOG: ... Warning: DKIM status for anonymised.ac.uk = none ()

So it looks as though it doesn't think there are any DKIM signatures in
the message.

I would be very grateful for any clues as to what I am doing wrong.

Thank you

--
Richard Gilbert
Corporate Information and Computing Services
University of Sheffield, Sheffield, S10 2TN, UK
Phone: +44 114 222 3028 Fax: +44 114 222 3040