I find a Speedtouch 585v6 for ten quid with free postage, so I'm in like Flynn. Box arrives three days later, and sho' nuff its a new router and although with euro plug it also has a UK adaptor.
I plug it all in and check it out. No connection to anything, no great surprise there, so I go for the set up wizard. Ah. The menu is all in Spanish, although if you put the wrong thing in a box it complains in english (very much like my trips to Spain, in fact), and there are very few options available - PPPoA or RFC1483 - and it seems that manual configuration is not possible, the wizard has to do it. Ah well, its off to the web to get more information.
Several hours later, the best advice comes from the zen web site, who advise replacing the user.tpl file (this seems to be what drives the setup wizard) in the router with a new one which offers more in the way of options. Long story short, I now have a working router although it only works if I make my IP address public and don't use NAT - this requires me to configure each of my PCs with a public IP address (I think) and I'm not sure how that will complicate my life.
However. I have a new working router for a tenner, and will spend a bit of time experimenting with NAT and other stuff. Stay tuned