Today I discovered another producer of diamond twill wool fabric--Ardalanish Farm. Ardlanish is located on the Isle of Mull. The Ardlanish weavers produce a number of different wool twill fabrics, but the diamond twill is woven from fawn brown Manx Loaghtan and dark brown Hebridean wools, and appears from the photograph to be a lovely reddish brown color. The page where this fabric is sold can be found here. Judging from the ruler scale in the photograph, it looks to me as though the thread count is about 10/10 per cm and the diamond motifs are about a centimeter across.
Unfortunately, it sells for 55 GBP per meter. I don't expect to be buying any of it anytime soon, but I like knowing that it's available.