Maybe you can advise me on what I should do.

On Sunday I parked on St Gile's Street in Oxford. I went to
pay at the machine and saw that it was free on a Sunday, but
that I needed a ticket. I therefore pressed the button and
the machine came up with "Machine not working." I pressed
the other button and nothing happened. I noticed the other
car parked there had a display ticket but assumed the
machine had stopped working since they had had used it. I
therefore decided there was nothing I could do and as it was
free on a Sunday in any case, I didn't worry. Imagine my
surprise when I came back and found I'd been fined!

I do not see why I need to pay this fine. I should not have
to pay a huge amount of money for something that was free.

I have written to the council but I don't think they will listen. What should I do? What happens if I don't pay?