South East Finecast do a very good sheet for corrugation - they also do the square shaped cladding ( as used on modern buildings like modern engine sheds. They also do a clear glaze corrugated which fits with it. Not only is it formed both sides but it is flexable and thin as well and far far better in both scale and detail than the wills sheets that are far to thick to really effectively use for building anything.
FBS409 Corrugated Iron Sheet 2.95
FBS410 Corrugated Clear Sheet 2.95
FBS413 Profiled Steel Cladding
For some reason it is not on there online site except if you go on to this PDF ( page 7 ). I rung them recently to check they were still making it and the guy said they were and you can order it at 2.95 a sheet.
Hope this helps you all.............