Lots of credit to the Support Ticket team, they responded within hours, initially suggested:
To upgrade your Evernote account to Premium through iTunes, take the following steps:
Log into Evernote on your iOS device.
From the “All Notes” page, tap the Account icon (the person icon in the menu bar).
On the following page, tap View Upgrade Options.
On the “Evernote Premium” description page, tap the desired payment frequency space at the bottom of the page.
On the “Subscription Details” page, read the payment details and tap Continue.
On the popup sliding up from the bottom of the page, select Confirm.
Enter the password for your Apple ID and select Sign In.
This didnt work for me, and it appears my downfall is paying annually for Evernote Plus through Apple
"Since you are subscribed to Evernote Plus through iTunes, we will need to wait for your account to downgrade to basic first which is on June 18, 2021 (based on the screenshot), before you can subscribe to Evernote Premium."
So I would say this is resolved, but not particularly satisfactorily, pretty disappointed to be honest 🤷♂️
NB Evernote Plus is a legacy product which as I was paying each year kept rolling over.