On advice from the experts on the pedelec forum, I opened up the two-speed motor again. Before, I'd put the improved grease in like spreading butter thinly on toast; this time I whacked in great dollops, like marmalade on toast. Or chopped liver.
The rim arrived today. I measured it, and it was exactly as advertised, so I did the calculations for the spoke size, and ordered 36 spokes from probably the best place in the UK to get spokes. I'm getting 36 spokes of "Sapim strong", because apparently those are the strongest spokes. I'm also getting spoke washers because I think the holes in the hub are a bit large, and a proper Park Tool spoke key (I've been using an el cheapo tool up tilll now).
I think I'll be able to install the rim, spokes and hub togather, but if I can't Tiller Cycles will do it for me for £25. I'd rather do it myself though, because I do like making things.
Also the tire arrived today, it's my usual Schwalbe Black Jack; reinforced with Kevlar.