Conveniently I can no longer find the bit of the allotment rules that says No Bringing In Gravel, so I am making plans to blithely bring in gravel for the sake of a base for The Shed, which is Definitely going to Happen this time, Honest.
The chief component I am now missing is a floor. Conveniently, there's an almost-complete house being built just up the road, with a big skip outside it, which currently contains several large sheets of plyboard. I can't actually get at them (it's all behind gates), but I am intending to show up on Tuesday morning and look hopeful at whoever's working there then.
(I am also missing enough sharp sand to level, and the gravel, but gravel at least should be fairly readily acquirable. It is possible I am also missing Some Important Bits Of Wood, but I care less about that because I have so many bits of misc wood at the allotment that I am pretty sure I can cobble something together.)
I am not going to manage to get all of this together before I disappear off to a field for a week, but I'm optimistic about getting it done in time to e.g. actually fill the greenhouse with chillis for the summer (an irritating amount of said greenhouse is currently functioning as storage space and actually I'd prefer it to be growing space. Actually.) Even I have now read enough guides to putting sheds together that I'm at least half-convinced I can probably actually more-or-less work it out.
... I will report back either triumphantly or shamefacedly in a few weeks' time. Watch This Space, etc.