Thanks for your pointer, big-all, though my mistake was in fact to use an ambiguous subject line that coincidentally matched other posts. The first post was indeed intended to be a new post commenting on the page, not a reply to any of the previous posts. Lesson learned.
Thanks for the information on the scarf joint, Dewy. The photograph I saw was nowhere near as interesting. There's a similar joint in my loft on the deep narrow timber that runs along the apex (I don't know the name, but if it were a tent it would be the ridge pole). That one differs slightly in that rather than a zig-zig there is a large notch in each piece to create a square hole. Into the hole are driven two wedges from opposite sides. Instead of using glue, or to add extra strength, the thick end of the taper goes back at the reverse angle, and the thin end has a matching point, a bit like a spliced joint. More succinctly, it could be described as a wedged scarf joint with opposing spliced joints at each end.
Cheesy