I had another go at a comments feed which would be a complete one — i.e. it would show the full post and also each of the comments, with one channel for the whole blog, and each entry and its associated comments forming an item within that feed. I struggled to produce something myself, but couldn't produce something that actually worked.
After a Google session, I found this lovely template on etc, tried it out, and it works just as I had hoped it would.
So, I've provided the new feed here, or you can click on the 'RSS Post+Com' button in the sidebar. You don't need to subscribe to both the standard RSS 0.91 feed and this new one — just choose one depending on whether you want comments or not. I've left the individual entry feeds too (at least for now) — never let it be said that I don't offer choice!