"Over payment of car park tariff"

The radiology department was very efficient and I was seen straight away and out within 15 minutes. When validating my car park ticket I was charged £1.70 and on examining the ticket after paying, the time stamp verified that I had been under 15 minutes which qualified for " no charge" . I then spoke to a man through he help button who refused to refund my money. He explained I should have proceeded to the exit barrier without paying but this is not highlighted on the parking tariff notice. When I pointed this out to him he said that I should have used the help button to verify the time I had spent in the car park, thus changing his mind on the correct procedure to adopt. He placed the blame on me for paying, despite their being no clear guidance on the necessity to calculate the time spent/tariff in the car park oneself. I wonder how many more people have fallen into this trap assuming the machine will correctly work out the charge payable and assume no charge if under 15 minutes. It is very misleading and fraudulent and the machine should be changed to rectify this or a notice to the same effect.