Ok, I now have my 14 sheets of Onduline corrugated roofing ready to fix in place. I didn't end up getting a price match from Wickes because they demand a receipt from another shed for identical material, B & Q do Coroline, not Onduline and Fred the Outside Shed man at the local B & Q seemed to only exist as a telephone contact for the other staff rather than someone with whom to press the flesh.
So Wickes it was. I am certain I need to screw the roofing on over the existing felt rather than use nails. The screw plus washer packs in Wickes have 55mm screws which is too short for Onduline, and don't sell the capped washers separately. I've searched online at B&Q and Screwfix, but can't locate suitable capped washers. There seem to be some things called Selawashers advertised on the 'net, but is there a more generic name for these things? I suppose I could use penny washers and gunge, but something squishy would be better, I think. I'm going to need about 300 and I really prefer to be able to see and feel the goods rather than mail order.
Why is everything so difficult? I suppose it must be me.