Plastic corrugated (flexible) conduit. Was the remains of the ventilation system left over from some building work.
Dimensions 72mm outside diameter, 60mm inside. Length in excess of 35m according to my guestimate (which could be wildly out).
Not actually as clean as it looks in the pic, but will certainly brush up nicely again after a wash.
If you're interested please say when you can collect.
Flexible ventilation tube/conduit (Upper Wolvercote OX2)
This offer has been gifted to someone and is no longer available.