No, they haven't got you 'bang to rights', because the intrusion into the YBJ is trivial, and therefore could be considered as de minimis in law. The only problem is that de minimis is subjective, so an adjudicator may not agree with you. In addition, you would have to risk the full PCN penalty if you take them to London Tribunals. However that is your maximum exposure as there are no other costs at all at LT. Of course, if you win, you pay nothing.