It turned out I didn't have it all right after all. The comments had gone again by this morning and the only seemingly helpful piece of info I could find online suggested that I should ditch my custom blog template and start again from a standard one.
Hence the new look.
It worked, but only for as long as it took for me to work out that I was now back to standard Blogger comments and I had to reapply DISQUS comments. Once I'd done that, the comments were gone again.
So I sent a support request to DISQUS.
But later this afternoon, I realised that the blog seemed to be trying to redirect somewhere, and mostly failing. After a short while it became clear that it was trying to redirect from a .com domain to a .com.au one. A little bit of searching turned up this little chestnut from Google. . It looked likely that Google had moved my blog to a .com.au domain sometime after I started this exercise.
It took a little bit of extra work, and a bit of fiddling around with the DISQUS "migrate threads" feature, but I think all the comments have turned up now. The redirection seems to be a bit flaky though, and sometimes it stays on the .com version, where the comments won't appear. Everything else seems okay there though. So if you're stuck in .com land with no comments, try this link to my "Australian" site.
Oh, and I think I will stay with the new look for the moment. I quite like the blue. What do you think?