These 2kWh 8s BMW modules from batteryhookup.com look very tempting, even with shipping from the USA. They are relatively cheap and light weight (13kg), and four of them in series would give me a useful range of ~80 miles.
This is the most promising battery option I’ve seen so far. The light weight means they’re only rated at 350A (5c) for 10s, but that ought to be enough. Four packs would come in at only 52kg, and they’re compact at
368x178x102mm