Can anyone fill a gap in my fractured memory, its a UK childrens Adventure feature or short film mid or late 1960s ,it screened several times at my school in the late 60s or early 1970s, i remember i really liked the film but can only recall the opening scene = Kids watch as a grumpy man lays cement (concrete) with a trowel on the top of a brick wall, then adds broken glass, assuming to keep kids from climbing over the wall.