Hi, You were headed in the right direction, the reason for my confusion woke me at 4a.m. -
I installed a fresh 2.3.4BS & and updated the PayPal App. Confusion arose because although the site admin showed the app version v5.018, running a file compare of the site pre & post update showed no differences.
Unfortunately, (you may be ahead of me already) the site compare was run on my PC scripts but I hadn’t downloaded after the update! At some point after the update I'd uploaded paypal_standard.php again (can't remember why).
Thank you for your time & help with this.