Having ascertained that I can build a shed for an estimated £50 + a few weekends of cursing and ingenuity, I began to look askance at the £300 being asked for a polytunnel. Hmm. Turns out it's easily done with 6x2m scaffold tubes, & about 40m of 50mm mdpe gas piping. Improvising a the polythene cover would be possible, but I can buy a ready made one with a door etc for less than £50 on eBay. So that's it for less than £100.
All of which means, this coming winter I can have both my shed and my polytunnel for about £150 the pair, that's 25% of what they would have cost me from a commercial retailer. Meanwhile, I keep myself firmly on the right side of the allotmenteers' ethical code by doing it all on the cheap, by the sweat of one's brow.