CL charging more than advertised - what do you do?

We had our first negative experience in a year of caravanning last weekend. We had booked a site advertised at £7.50 with EHU.  We booked the Fri and Sat, confirming it was ok to depart later on Sunday and confirming the price as it was so inexpensive. We confirmed it was £7.50 and actually later phoned up to book the Thursday night to site the van then leaving us more time to enjoy the weekend as it would still cost at only £22.50 or so we were lead to believe. However, on departure we were told the site was actually £12 per night (62% more than advertised). My partner stated we had confirmed £7.50 on booking but when the female owner turned up she stated she was aware the CC Website price was wrong and had told the CC, but she told everyone the correct price on booking. We explained that it was a gentleman we had spoken to and she stated it would have been her partner or son. She then stated she would take £30 and my partner then paid it, coming back to see me most upset. This is the first time this has happended to us and it really spoilt the weekend.  Talking afterwards we agreed that if it happens again we will pay what we had agreed over the telephone but happly leave our details. What would others do in this situation? (Incidentally, I emailed the Club on Sunday night and by Monday morning the correct price had changed on the website)