But the way that they dealt with the whole thing smelt very "we don't know what we're talking about", enough to put us off.

And shifting all the IP space about would have had costs with very little return, so little business appetite to go through it.

I’ve seen this commonly from them with residential service. They mostly give you a private v4 address behind CGNAT in addition to IPv6. If you want a public v4 they put you on their old network which has no v6.