However, I would like to save for posterity how I managed to get it working.
Importing the articles proved impossible as I couldn't give it an RSS feed and let it do the rest, which was a shame. The utility of that would be immense. So, I ended up copy pasting work in and then stripping the formatting (after realising that it had kept the background colour of the text the first time I did it). It did give me a chance to go through every article and add edits, fix links and so on (desperately trying to find silver cloud here).
The main problem with migration was the use of a custom domain, or rather, persuading Blogger to accept that I had authority for my existing domain.
What it boils down to is that to make it work I have the following zone entries:
(my verify key) CNAME (my verify token).googlehosted.com.Where (my verify key) is the CNAME key and (my verify token) is the CNAME domain they want you to add in the domain authentication settings.
www CNAME ghs.google.com.
The most critical part of this that mattered was the inclusion of the full stops after the two CNAME values.
Basically, the first page of instructions, the one on the Settings->Basic->Publishing screen, and critically the one you see when you enter information which hasn't worked (giving the infamous Error 12 / Error 13 messages) - has the wrong information. It advises putting the address in without the full stop.
If I was more clear about how to put DNS records together then I probably would have spotted this sooner. As it is, it took until the next morning when I stumbled across a different page - the one that is linked as "settings instructions" and tailors its information based on what registrar your domain uses - before I noticed that it included a trailing full stop in the domain verification value.
Once that was set up, it all went quite well. The "missing files host" facility works really well, so I can point that back at my old server and it picks up any files that are not present on the new www host.
The site looks a lot better. And I don't have to run a really contrived set of operations to add an article any more. Though, in all fairness, I did *write* that contrived set of operations. My bad :)
I've isolated what the confusion is. The set of instructions that appear when you first type in your custom domain, where the operation fails with an Error 12/13 - THOSE INSTRUCTIONS ARE WRONG. Then, the linked page called "settings instructions" claims that you need a full stop after the name of the verify token (which is correct), but doesn't include one after the "ghs.google.com" address - THOSE INSTRUCTIONS ARE THEREFORE WRONG.
The only page with the correct information on is the "webmaster's verification tool" page which you have to click around to find. The CNAME method verification page in that area has the full stops added to the back of both the www / ghs.google.com and verify token addresses.