Well, fair play to the engineer. He really did phone around some colleagues for me, called me back a few hours later, and managed to get the fault fixed. We're online as of this morning. (He wasn't getting anywhere with their official call-centre, I think often times the engineers are set up to fail a bit! I've seen brand new ONTs thrown in the bin previously, because activating a new one was easier than getting through to someone competent on the phone.)