[Exim] ClamAV & Exim 4.32

I have just set up ClamAV and Exim 4.32, both running on the same box. I
have an email client (that shall remain nameless!!!) that is repeatedly
sending a virus-infected message to Exim. 50% of responses are 451 -
Temporary local problem (with Exim reporting "malware acl condition: clamd:
unable to read from socket (No such file or directory)") and the other 50%
are (correctly) 550 - Malware... Detected (Note that the client doesn't give
up on receiving 550!!). I presume that I've got something wrong in either
the Exim config or the Clam config. Any ideas?

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David Allen
Manager - CMC Systems
Information Services
Queens University Belfast
Belfast BT7 1NN
Phone: +44 (28) 9027 3886
Email: d.allen@???