Thanks guys, I wouldn't have considered the back pressure so well done. I bet neither of you would have read the thread if I hadn't made the title so mysterious.
Probably still very boring though. We're planning an extension on the side of our house where the tumble dryer vents currently sits. It would make an extraordinary difference if we could leave the tumble dryer where it is, but that means it would be venting into the new lounge which isn't...