Hi Folks.
Well, that was embarassing, I fixed the bug in the receipt code and promptly introduced another one!
PayPal require some updates to the code which exchanges payment information, so I rewrote it, ran a test on the PayPal test facility (the PayPal "Sandbox") and everything worked as it should. So I substituted the old code for the new...and it failed, issuing FOPS members with the message
PDT Failed
There was an error. PayPal indicated that the Payment Delivery Transfer has failed. If this message is displayed after a PayPal payment has been made it does not necessarily indicate that your payment was not received by the FOPS.As the message says, your payment has been processed correctly and entered into the ledger, but you didn't receive a receipt from the FOPS website.
I have temporarily restored the old version while I run tests on the new code
Sorry about that, I will post updates once I fix it