Thanks for all that: I think it confirms what I'd suspected about the medium and the message being the two different things. It also seems to have a parallel with ISPs and porn: the ISP is providing a legitimate service: but it also has a duty to prevent its being used as a channel for porn (and terrorism, and, and...)
Anyway, my daughter's pretty sensible and now she knows it's illegal, she has voluntarily deleted the program. (I haven't yet asked about the few dozen songs she had already downloaded...)
It also seems that the BPI are distinguishing between those who upload (i.e. provide files for sharing) and those who download (those who partake.)
btw what's the difference between peer-to-peer and hub based?