OK…I needed to upgrade and have done so.
I still get the email with an error message instead of the download link. The error log is still blank.
Have have debug checked, the secret word and url in place, downloadable product checked and the SKU code in the “Additional” section.
A standard paypal button with IPN in wishmember works, so suspect that there isn’t a problem with server settings.
Can you think of another mistake I could have made?