After several hours of trying different configurations the OTA EPG suddenly started to work. I'm sorry to say that I cannot explain exactly why. I must have changed or reset something - a filter perhaps - while I was running through the options.
It has successfully downloaded, and populated, the TV Listings three times and is currently running another background task. My feeling is that it does download the data but did not extract the program listings successfully. Now, mercifully, this appears to be happening.
I'll post more details if anything else happens.
John