After the problem with the card, I rang and complained and Virgin Media agreed to send someone out with a new card to install it for us. The guy arrived yesterday with no idea what he was having to do. Fortunately though he had a spare card on him and installed it, also switching Audio Description back on for us.
So the saga ends. I’m not entirely pleased with the level of customer service we received but the job has been done and all is now working correctly which is the main thing.