Ah, yes that was the trick! My old Mastodon account now seems happy to migrate, which was the nub of the problem. Thanks.
Also once migration has started from Mastodon there is no longer access to the old account to export followers and followees (fortunately I’d already done that). Do I need to reimport the people I’m following? There doesn’t seem to be anywhere to view the current list.
Perhaps these could be noted as a step in the help page for custom domains and migrating from Mastodon.