What case?
Not following a code of practice isn’t illegal!
However you might have grounds for a complaint to a useless and toothless trade body, such as the BPA. That might get you a POPLA appeal, perhaps, but then on what basis would you appeal? The driver overstayed, you say, and if the signs are adequate, then what?
However if you wait for them to take you to court they’ll eventually give up, if you follow advice here.
“Nobody pays a penny to ECP if they follow advice here.”
Have a read of similar cases, I suggest.