OK, managed to sort this.
To sort this I ran the CAD postinstall.exe at \Prog Files\Cisco\Desktop\bin on every agent PC.
Normally you only have to run this if the PC has multiple NICs (mine just have the 1 built in NIC). It looks like the PCs where imaged with different network settings which still kick around in the registry & when CAD is installed it's bound to these settings instead of the correct network settings of the active & enabled network!
By running this util you force it to bind to the correct network settings.
Maybe this section of the install guide can be updated to read 'If the PC has had multiple networks configured...'.
I'd recommend to always run this tool after CAD setup from now on even if the PC does just have one physical NIC.
Cheers,
NJ.