The good news is that my parking infringement notice has been WITHDRAWN!
A week or so ago I sent another polite letter requesting clarification on the following point: that since they had acknowledged the existance of the sign I sent them a photo of, how was it possible that there should also be a sign stating restrictions on Sunday. Could they tell me exactly where it is please?
So now they have sent me a very polite letter withdrawing the notice. Yippee! I win!