"parking fine"

Have just received s parking fine for £85 from Trowbridge hospital? I normally park at the front but was no spaces so I drove round and parked in the back of the hospital, my daughter was having a tetnus jab she is over 18 and said she would go in by herself and I would stay in the car, was lots of spaces around she returned 20 mins later and we went home. Two days later I get a letter with a fine for £85 or pay £40 if within 14 days . I didn't read the sign or thought I had parked in a wrong bay? Was unaware of having to register my car inside? As I never entered the hospital and stayed in my car ? How can I avoid paying such a hefty fine for being a kind mum taking her daughter to a hospital? Please advise many rhanks