Do the owners also own the car park? Are the spaces owned by or allocated to each flat?
They have they have absolutely no legal basis to fine you for parking in your own parking space. As you will appreciate the time you take to reply is irrelevant as there is no case to start with. The solicitors won't have even thought about the law here. They just send out letters based on a lost of names. Write to them pointing out that you own the space. I hope that helps. Please leave a positive rating so that I am creditd for my time.
I will be here if you need my further help.
Take the papers to a solicitor and make sure you defend the action. You can't be charges for parking in your own space.