I connect to bt broadband (adsl) through a router. The dns-servers are assigned dynamically via dhcp - which seems to be a problem. While some pages load fine, others (most prominentely google and ebay) are impossible to access. I've heard from friends that they experience very similar problems with a variety of different routers and linux-flavours, but always involving bt broadband. Today I've come across a German website where they've published a long list of valid T-Online dns-server addresses. I have added one of them to my resolv.conf, disabled dhcp and suddenly all problems were solved. Now I wonder whether someone knows of a similar list of valid bt dns-servers for britain.

Try using one of the public root servers:
http://www.public-root.org/root-server-locations.htm