I HAVE FIXED IT!!!
I had a hunch this was an operating system issue with ISPConfig not set up for the right O/S.
All I did was to re-install ISPConfig2 on a new server (fine - I had 2 to confugure anyway), and copy it's isp_server table across to the broken server, and then I just edited things like the IP addresses and hostnames. Restarted the ISPConfig service, and it now works as it should. Thanks for the tip