Not really a Pi problem. The Pi controlling the Robotic Arm stopped working; it wouldn't even boot up. On investigation, this turned out to be a failure of the SD card. Fortunately, I have an image of each Pi's SD card on another server as a backup. So I used that to create a replacement, and the Robotic Arm is operational again.
Hobbyking got back to me to tell me that I could only have the "buddy price" for three batteries, and I wanted eight. But by then, I'd worked out a way to get the eight at the price I'd paid for the original four, so they're ordered and will probably turn up some time next week.
I'm bidding on Ebay for another Haro folding bike. Because they really are very fine, and I'm hoping I can get another one for £60. It'll be good for spares, although I have a feeling that I might turn it into another electric bike :-)