Den?

I have been walking by this sheet / cover up in the tree for years. It's looking quite battered now but used to be cleaner and tidy looking. I originally thought it could be a den or hideaway as it's by the school but who knows. It's survived all weather's so must be secure. 
A flurry of snow tonight but nothing more. 'A' was at a meeting near Gloucester and was certainly battling it on his journey today.