The various bits I ordered from Ebay arrived, so I put in several happy hours on the bike. I've installed the back rack, and it seems to be firmly seated - I used one of the brackets that held the battery to the seat post as a bracer (I won't be carrying the battery that way on this bike). It'll hold the controller, and it'll take the panniers that hold the batteries, and there's a quick-release so that I can pop them off to get over an obstacle (with the panniers containing batteries and drinking water, I can barely lift the whole thing, but with that taken off, I can).
I installed the front brake, replacing the disk brake that wasn't going to be possible with the motorised wheel in place; the one I bought was quite cheap (£5) and came as a pair, so I have a spare. That seems to work fine.
Then I replaced the gear shifters and brake handles with much nicer Shimano combination shifter/brake handles. This takes up less width on the handlebars, which is necessary to accomodate the throttle. That meant I spent a long time adjusting the gears and brakes (and that still isn't finished). But that adjustment needed doing anyway.
And I've ordered a 20AH, 24 volt battery from the folks in China. That's twice the capacity of the ones I normally use (and twice the weight), but at a much lower price.
Still to do:
Finish adjusting the gears and brakes
Install the PDA bracket
Bolt the motor controller to the back rack, so that it can benefit from air cooling.
Tidy up, and test.
... update ...
I've got the gears and brakes adjusted now, and I've fitted the PDA bracket. I'm going to replace part of the brake cable, because the run I'm using now is a bit too long. I have to think about what will be the best way to clamp the motor controller to the back rack.