Do make sure the polythene sheeting is UV stabilised. Spent hours and hours recently picking up tiny fragments of polythene that had broken up from a temporary coldframe built last year, and like an idiot I used the plastic sheeting sold in Wickes for use when decorating - it just isn't suitable, would have been ok if I'd taken it off before winter I suppose, but left it and the lot broke down very quickly over winter and what with the winds, it scattered over quite a large area on my plot.
