Looking at the logs, the payment was completed successfully and returned to https://www.blahblah.com/index.php?m...eckout_process appended with a list of parameters and values confirming the success of the payment request (shown below - this was returned for Order I.D xxxxxxxxxx

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In terms of the error message, as this appears on your site, it would appear that there is an issue with how you are handling the response from ePDQ.
This was their response. I guess I need to look at my site in more detail.