Hmm. I don't know if he's exclusively their designer, but in all three of the books I have, he lists Rowan yarns for his designs. Though in some cases he doesn't specify a yarn, just a weight and tension.

Glorious knitting is wonderful. The pattern shapes are rather boxy, by and large, but there are some really inspiring colours. I've knitted several of those. And did you see turlough's Kaffe Fassett throw?