I made a guess. it turned out correct:
Trek/Bontrager (Select) Freehub Body 561948 (repl W268886)
Available from Pedalon (where I ordered from) or GB Cycles (from whose catalogue I figured it out).
GB Cycles listed a few similar alternatives but they can be discounted:
- 561949 is 11-speed (although worryingly listed a 130mm OLD, whereas my bike is 135mm OLD - but there are no sizes on the 561948)
- W268886 has been superseded by the 561948
- W211339 has sealed cartridge bearings
I'm still waiting for Boardman to come up with an actual part number or Halfords reference, although it's a moot point now.
I've only actually done a quick test fitting tonight, but the freehub fits the existing hub and the cassette fits on the freehub, so I think I'm good.
Not a direct answer, but I called up Boardman. Not entirely surprised they didn't have a part number, but they were able to offer up a reference with which I could order from Halfords. They have me the usual runaround, but hopefully should have something that fits middle of next week. In the meantime I guess I'll continue getting fat.
Boardman's reference on Halford's system: 341740
Receipt description: "FHB CB/Car/VD Splin F th"
Except it turns out that doesn't fit. Different gender. Drat.