it seems to have caused "problems" with the Technicolour router), but the Zxyel one seems to now be working...
I just had a thought, the first time I attempted to connect, the network icon in my sys tray told me that I didnt have an internet connection.. when I really did. I think Windows gets a bit confused if you swap between a combined dsl router and a router with a separate modem. Its probably something to do with how it sees the network routing.
If you go to the network troubleshooter and let it search for and fix problems, that may solve it. However I actually made a separate network connection from scratch, so I now have 'Network 1' for the TG582n & HG612 modem and 'Network 2' for the ZyXel.