Ok....
Update:
I've relaunched the site - spindiggity.com. Ive gone through all the pre update checks for Joomla1 4.2.6. The only extension
That 'may' cause an issue is the S5_Flex Menu. It's version I have loaded up is is new than the one it wants me to upload? So I turned the S5_Flex menu off, now it doesn't show as a potential migration issue. (Shold I remove it completely?)
I have downloaded the new template, Outdoor Life.
I then tried the migration - The admin side works. The front does not.