Gladstone Solicitors - gym parking

I received a parking fine in the post in November from my gym.

The parking ticket was actually issued whilst I was in the gym signing up and having an induction. I didn't notice any signs in the car parking and it wasn't pointed out to me on arrival. I was sat with a member of staff to complete my signing up forms for half an hour and whilst they didn't mention parking they did happen to mention they were still drunk from the night before and to not get too close!

I got a parking ticket within a few days of signing up and then realised that there is a small machine in the gym where you need to add your registration plate and it then gives you unlimited use for 30 days.

Stupidly I ignored the first and second letter from the parking company as I believed that calling or writing to them would admit my liability and that it wouldn't go any further! I did bring it up with the receptionist on duty when I left one day and they told me to ignore it as they never chase it up anyway. (they wouldn't have known my name as I asked when leaving and they don't have any photos on your membership or thumb scanners when leaving).

I've now had a 'letter before claim' letter from Gladstones solicitors on behalf of Sip Parking that says they have been instructed to commence legal action against me. They refer me to the Practice Direction for pre action conduct under civil procedure rules, paragraph 13-16?? It states I have 14 days to pay the full amount or to acknowledge receipt of this letter and provide a full written response. The response must include the name of the driver and their address or they will start proceedings against me.

Have I made an expensive mistake by not responding? Should I contact the gym head office first to see if they can cancel it or do I respond to Gladstone? What will they do next??

I did try reading the sticky but got a little overwhelmed with all of the abbreviations and think it confused me more than I was before!! Any help is vastly appreciated.

Thanks