I'm not sure about MM being "by some margin" the most famous novelist to write a Doctor Who novel.

You could be correct. I suspect I'm being biased here by my age and social group. I recall Moorcock being quite a byword among the people I know who were interested in SF when I was a teenager, and I notice his works and characters getting referenced a lot in genre fiction and fan fiction. I'm much less aware of Baxter and Reynolds just as a part of the background noise of fandom, but that doesn't mean that they aren't more famous in terms of the wider public and book sales.