Me again.
Apparently it's a 12 ton Box Van. BR Standard Goods Van. Made in high numbers in the 1950s and some were used until the 1980s. Not so common now but don't change hands for much money.
http://www.scalefour.org.uk/parkside/PC07A.gif looks about right (although just a black and white sketch of a model!).
To the guy who asked about the floor, I presume there won't be one. There must have been about 5 foot of leaves on the inside and if that's been repeated every winter for donkey's years then imagine it's completely rotten.
Cheers
Pinsanon