What an amazing bit of film! I knew that WW2 tracked vehicles were recycled as agricultural tractors, especially in Australia where many Valentine, Matildas, Stuarts & Grants were modified to varying degrees, but I never knew WW1 tanks were used in a similar manner.
Great find, Helen; shame someone didn't think of using tanks like this in Britain - many rhomboids could have been saved from the scrap metal drives of WW2.
Surplus WW2 tanks quite commonly used in Australia for scrub and forest clearance by farmers - http://commons.wikimedia.org/wiki/File:040tank.jpg - and contractors (latter often used them in pairs with a heavy chain between them, sometimes with a sort of "demolition ball" in the middle) but rare, I think, to find evidence now of WW1 types in such service anywhere. Well done, Helen!