I wonder if something went wrong with the investigation here, they looked at the wrong item or got the wrong evidence bag. If there were pieces of porcelain in the bag - and the porcelain wasn't discounted for some reason - it's difficult to see how they reached the conclusion that there was nothing wrong with the food.

In your shoes I would ring Customer Relations between 14:00 and 16:30 hrs UK time and see if you can get someone to look into this in more detail. Alternatively start a DM conversation via Twitter.

I think the CRA process may still work but that isn't going to get you an apology very quickly.