Had to reset and reload all books to get round this (couldn't find another way)
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However I did find, on the Kobo help pages , a reference that ADE version 3.0 may lead to less authorisation issues when loading e-library books than v4.0. There was no explanation why, but to try it I have from 4.0 to 3.0, reloaded the books and will see how that goes.
Dave