What the DVLA say and what they can actually do in LAW are 2 different things.
1. They CANNOT fine you as there is no statute law stating this, however they can issue a demand the same as you, me or Bellend. The fact they are a government quango means nothing.
2. DVLA cannot make you contact them if they mess up and they don't recieve / lose your letter and the system isn't updates.
3. DVLA cannot alter statute law to suit their own interpretations ie Interpretations Act.
The DVLA have no teeth at all when you have followed LAWFUL procedure and not one they have made up. They cannot fine you and the only way they can get money back is due to a court order. They thrive on people getting scared by their long words and monetary demands. If certain people want to run like sheep to a rogering with them then carry on.
As soon as you post that letter, that is all the law requires. Full stop. Court after court keep finding the defendant innocent, what more proof could you want