Hello, Thanks all for your comments they were enlightening. I'm sorry that I haven't replied sooner.
I now think the issue was peculiar to the client's PC and network configuration. My hypothesis is as follows: The security configuration on the PC concerned requires a login to a proxy before allowing access to the Internet. When the Open Toolkit looks for the internet it is served a login dialog which means that it is somewhere between an outright failure, whereby it would use the DITA catalog file and successful reception of the DTD from Oasis. This would also explain why it was working when we installed it on the client site (the Internet connection was logged in) and failed when we returned a week later. Thanks again, Richard
