This caused memory to very slowly stir, and I eventually remembered that somewhere I'd seen some Mr. Benn / Invisibles fanfic.
And eventually Google found it for me....
Mr. Benn: Invisible by dontmakeeyecontact
http://www.geocities.ws/dontmakeeyecontact/MrBennInvisible.html
Worth a look if you like the Invisibles (or like Mr. Benn and can put up with a VERY strange adventure with lots of swearing, gratuitous violence, sexual ambivalence, etc.)
