What exactly is the legal position regarding proxy servers etc? Are the server owners technically responsible in legal terms if their server is used by someone else?
Yes, the owners (or rather the person who's name is on the internet feed) is responsible. They'll knock on your door, serve you a warrant, interview you under caution, take your servers and do a forensic analysis on the logs etc. If you are not fortunate enough to live in a "Western" country, your treatment may well be substantially worse.
And to the OP, as SASless said.... if you piss someone off enough, they'll put enough resources into hunting you down. So to think a proxy, onion routing, or some other service is going to protect you, you are much mistaken.