I've looked around for a fix, and done the following...
-ensured clamav user is in the Debian-exim group (standard first step
for this problem)
-chmod'd recursively on the directory in question and added write
access recursively for the group
-found something about AllowSupplementary groups in the clamav.conf, did that
-after all changes i've restarted the services
This in om a newer Debian install, exim 4.52, sa-exim, ummmm, not sure
what else is relevant.