Free parking but still got a ticket

[Brandonflowers123]

I got a ticket from a private parking company in a private car park.

However it was free from my calculations.

1. 90 mins free according the there signs, then after that you have to pay. So I got there at 16:37.

2. After 1800 hours it's free parking.

I've been doing this for a couple of months now, but recently I got a parking invoice.

What should I do?
Am I right with my timings?

I'm pretty sure I am in the right, so I want them to see if these private companies take you to court? And if they take me to court just laugh at them at court

Or should I play it safe, and call them?

Bearing in mind my job requires a clean financial check every year