Thanks guys, a friend dropped in to help with this problem and he pointed out that the modem has LAN IP address 192.168.0.1 (according to the manual), but my home LAN is all on the 192.168.1.x block. Therefore simply plugging the modem in will not make it visible to ping or a browser. This was the major assumption and mistake on my part.
So, as a hack we changed my workstation to be on 0.x block, then I could browse to the modem's config web. The modem's LAN address was changed to 192.168.1.1 and after a bit more fiddling around everything came good because we were all on the same block. So the worst is over and the LAN is working though the new modem thanks to putting everything on the 192.168.1.x block.
However I still have a terrible problem because the modem's port forwarding config screen contains a blank text box with no controls to add/delete ports. Without this facility I am totally stuffed and will have to revert to the ADSL modem, as I am publishign web sites which are being used for app testing. I have to port forward 80 (and others) to a specific server on my LAN. I'm going to open a separate issue for this problem.
Greg