Thanks for that, I can see what you mean now.

But your plan (an extra chamber at the west end of Zennor) seems different from what Roy is suggesting for Trethevy, unless I've misunderstood that. I think Roy is suggesting that the "existing" chamber at Trethevy was subdivided into two (by the sloping stone), whereas at Zennor you're adding another chamber beyond the "existing" chamber rather than dividing it into two.