Other users were able to resolve the issue by doing the following:
1. Right-click the data source in the Data pane, and then select Duplicate.
2. Rebuild all views that used the original data source with the copied data source.
3. Once all views have been created with the new data source, delete all old views and delete the original data source.
4. Save the workbook.
I forgot about this one.
I did receive that reply from Tableau but I was loath to rebuild all the work I'd done.
However I later found that it was an action pointing to a sheet that I no longer needed and as such deleted.
Thanks for taking the time to look into this though.