Plusnet broadband offer...

You may have seen the posters and TV commercials? Well, Plusnet just sent me a letter asking me to sign up with them. I have never dealt with Plusnet before and I am not on any public mailing list so where did they get my name and address from?

Incidentally Plusnet is one the broadband providers who provided customers' names and addresses to disgraced lawfirm "ACS:law" which eventually leaked onto the internet.