Similar thing happened to me in North Yorks - I asked them to refund my
costs - postage - time taken to write letter explaining why the parking
fine was not applicable - time spent on phone calls to the council etc etc
They happily waived the parking charge, but said it was not their "policy"
to refund costs incurred despite their error.
So effectivley N Yorks County Council are telling me they cannot be sued
no matter what they do wrong.

Well done for trying at least.