"If the car parking machines don't work, what can you do?"

My husband attended the hospital re follow up of minor stroke and further assessment. The appointment was very late, hence when my husband got back to the car the ticket had ran out. He followed the instructions and paid for the extra time. However the money was taken but no ticket received. He tried again and lost the fee again. We then got a letter demanding be £40 or £70 if not paid immediately. We complained, but they came back and said we have to pay. This is ridiculous. If the machines don't work, what can you do?