Free Traffic Legal Advice

I recently entered a car park belonging to Napier parking in Shrewsbury. Parked then went to read the notices re charges etc. When I realised the charge was a fixed charge £6 for the whole day I got back in my car back in my car and quickly searched on my phone for a nearby car park which I could pay by the hour as I was only visiting the town centre for an hour. A week later I received a notice of a parking charge for £100. The letter detailed I had stayed in the car park for 8 minutes.    I wrote to Napier Parking appealing against the charge, saying it seemed unfair as I had not stayed  very long and asked was there not a grace period to cover these circumstances. They replied to say my appeal had failed, since I had exceeded their 'consideration period' This period was not given or displayed on the parking notices.  I researched that parking in the Britain is governed by by the British Parking Association (BPA) and that parking companies are bound to offer at least 5 minutes consideration period and that they also have to offer a grace period of 10 minutes which seems to refer more to people who were late in getting back to the car park following their time period.
Is it worth my taking my appeal to the next stage (Independent Appeals Service)?  I believe I might have a case in that they have failed to display the information about their 'consideration period'. I understand the law of consumer contract says that consumers need to know all the required information in order to make the contract.