Free Traffic Legal Advice

Hi all. I'm just asking for some help. I don't actually live in the UK full time but hired a car on my UK drivers license. I picked up a hire car and then picked up my family from the airport. We then went to McDonald's in Mondial Way near Heathrow and took a parking ticket and ate in the restaurant. Whilst I was in there, there were signs saying you needed to validate your parking ticket so I asked a member of staff how to do it. She seemed very confused and told me that it was only free for two hours and I said 'I know I've only been here 20 minutes'. I showed her the time on my parking ticket and the time on my phone. I could identify the staff member of necessary. She then went away with my ticket for about five minutes and came back and gave it to me. I assumed it had been validated so exited to the car park with it. I've just received an email from Enterprise telling me that I've got a fine for parking at McDonald's Mondial Way. They have already charged me £35 and I'm going to get the parking ticket in the post I presume. I'm just really annoyed as I took all steps I thought were necessary to validate my parking. I don't really know if I have a leg to stand on as presumably the ticket wasn't validated. I have complained to McDonald's but they're obviously a massive company and will probably just completely ignore me. So I think I'm stuck paying what will be probably over £100 for doing nothing wrong. Is there anything I can do?