An update to my original post on this subject.

I have tried everything suggested except deleting the column btDisplayOrder in table BlockTypes, which was not recommended. This was suggested due to the necessary downgrading to maintain site.

Have tried manual update, with and without JSON swap, removed line from config/site, and of course tried the dashboard update. the site continues to read the 5.5.2.1 upgrade in upgrades folder unless the line in config/site is removed and the old 5.5.1 core is active as core version. (Clear?)

There seem to be a lot of issues with this update. I'm thinking they are site specific...

Do you think that the theme updates could cause this update to fail? Will it be necessary to add a new database to my server and start again? This is definitely not the option I wish to choose.

Thanks very much.