I've managed to get the wipers working.
The grounding on the wiper motor was a bit iffy so I did a proper job on that, cleaned up the connectors and removed a few random wires that looked like they'd been used on previous accessories (cut at both ends but still left in place). It didn't work at first, not sure why, so I took out the relay, tested for continuity again, ran the wipers without the relay in, then when I reinstalled the relay everything worked fine. I'm not a big fan of things inexplicably working but I'll take it at this point.
The wiring colours in the manual all seem to match but my physical relays and their locations are quite different (so for example the continuity tests in that manual are wrong for my relays). It should be enough to go on for me, but thought I'd mention it in case you run into anyone else with similar issues.