Sidewinder -
Given your location, I'd recommend you get in touch with Gordon Vigrass at Bussens & Vigrass at Downham Market for great corrugated / profile sheeting prices, s/h portal / tubular frames, fasteners, erecting etc etc. They also have access to top quality 'second' sheeting rolls, if that makes any sense!!
You could also consider a Romney shed - either 24ft or 36ft x whatever length you need. They are easy to double skin and insulate by putting good sheeting over older s/h sheets or a cheap new liner sheet, with or without insulation between.