Measure the size of your car park, and then measure the size of other car parks in the same complex. If your car park has dimensions for 3 standard size car parks I cannot see how they can object.
Also check what the council regulations are for car parking space (might be less than your neighbors size of car park), and if 3 council parking spaces fit within your parking area again they would have trouble objecting.
I don’t see how they can create a bylaw to discriminate against you if you are using the parking space for it’s intended purpose.
If you go to adjudication take photos of any items others are leaving in their parking spaces (rubbish bins, ladders, etc) to show that the EC are being unreasonable.
I don’t understand why you had disabled signs above your parking as this seems to indicate they were for disabled people and so common property. The developer hasn’t pulled a fast one has he and sold common property to residents against council regulations.