There was an interesting programme on Radio4 about 2 weeks ago on this, presumably with the same gentlemen. Can't remember the programme name, and have done a search on IPlayer, but couldn't find it.
Looks great fun, being paid to prowl through miles of passageway.
I'm not so sure about that part of the job that involves spending hours up to your middle in sh*t clearing out accumulated plastic bottles and builders' bags so the heffluent can flow freely... :-\