After my little bit of inspiration over the twin evils of a shed in the wrong place and a nasty little problem spot, there was no point hanging around really so I got out the tools and got to work.
First job - dig the whole area out to a spade's depth (getting rid of all that borage in the process) and put down a nice clean layer of weed suppressing membrane. Lo - the problem area is no more.
Next, after those nice people at Wickes delivered a load of building materials, it was barrowing a couple of inches of hardcore (actually the same stuff road builders use so it can't be bad).
That was the (relatively) easy bit done - then I had another couple of inches of sand on top of that:
...and the tricky bit was getting the horribly heavy paving slabs down. These were recycled from the paving slabs that were alongside the shed already, which seemed like a good idea until I realised how damned heavy they were. They were the kind you use for pavements - great for settling down and not moving anywhere without a small bomb beneath them, awful if you do actually want to move them somewhere. I did my best - it wasn't quite patio standard, but it was OK for a shed base, and looked pretty good by the time I'd finished.
Then the fun bit - ta-da! One minute the shed was over there - the next, it was over here!
The tarp is because we managed to tear the (already gently rotting) roofing felt while we were moving it, but no matter, we have another roll in the shed which we'll finish it off with next week.
I'm dead proud that we got it all done - but don't be deceived by the "before and after" pics above: this was a long, sweaty, difficult job, especially the moving-the-shed bit which involved a lot of swearing and entailed dismantling the entire building and reconstructing it over a period of several hours. However - it's also a job well done: we discovered a rat's nest underneath where the shed used to be, since they hadn't gone to all this trouble and had just put the shed on fence posts laid on the ground. Ain't no rats can get through this lot, I guarantee...!