It's an honest response, and it does recognise their mistakes, the main one being the mis-communication that meant you didn't stop at Milton Keynes. It doesn't excuse it, but Control is extremely busy with these kinds of difficult incidents, and communications can be missed or passed on too late.
Edit: I am really sorry, but I reviewed and strongly disliked my phrasing in my initial statement about Hemel and have to re-type it as it wasn't 100% factual in the way I wrote it. I don't want to mislead allotments, who btw has a great and positive attitude to their awful experience.
Corrected : For various reasons, Avanti services do not stop at Hemel. This will be down to things such as train crew knowledge, dispatch procedures, stop car boards etc.