Have any of you folks used these. I've been told some fare better with UV degregation than others...and some retain flexibility better?
Do you mean polycarbonate (PC) roofing sheets? The corrugated ones are generally rigid and brittle but can be shaped using a hairdryer to heat them - I don't know how they'd fare on a windy allotment site - they act very much like sails so if you were to create a tunnel from them they'd have to be closed at the ends somehow and very securely attached to the ground. I think it would be safer to try and construct something square based around a frame and the kind of polycarbonate that they use in plastic greenhouse panes.
Thanks Squeezyjohn. I'd seen these and thought very clever but wondered if I could do a taller version.
http://www.lavenderandleeks.co.uk/shop/grow/clocheborder-hoops/