I thought the problem was cracked - but IT IS NOT!!
I thought I was on the way to solving this problem, but in a republish operation I lost everything again, so I will be proceeding very slowly, I'm afraid, with separate back-ups of the HTML code after every stage. I won't put the commenting script back up until I have got the page HTML code back the way I want it, and this is going to take a few days. I had already put the comment code up briefly, but after I lost all the template data yet again there seems little point. I'm going to be in touch with Blogger again to report this continuing problem. My apologies. Please bear with me.
UPDATE: (31MAY04 0905 BST) I am cautiously optimistic I have now got my blog template back to the way it should be; I suspect there remain a few minor differences. Very importantly, I have off-line versions of every stage of the modifications I have had to carry out over the weekend, so if the problem recurs I should be able to be up-and-running again very much more rapidly.