Ok after some searching I seem to have found the resolve for this issue on my server.
It appears to have been a permissions issue on the var/clamav folder.
The default install apparently had the permissions on var/clamav as follows:
Group: input 
Owner: saslauth 
I had to change this to:
Group: clamav 
Owner: clamav 
Once I changed the permissions and did a
freshclam - update virus databases then everything updated fine and the high CPU usage disappeared.