Thanks for the responses.
To clarify, I chose to ignore the notices thinking I was following the advice in the linked thread, i.e.
this bit - although I now realise the advice was to appeal first, then ignore the correspondence if/when they reject the appeal.
My mistake, and I understand and am sorry for the frustration caused - I respect and appreciate what the members of this site do.
Noted that I can ignore debt collectors and not Letter of Claim from solicitors, or a Claim Form from the court. Thank you.
Thanks also for removing the letter, you are correct - I had forgotten to redact the personal details.