Looks like a pretty good worked example in a training session for adjudicators of what de minimisin a box junction means ! The PCN is money-grubbing PCN we see far too often. However the council, venal and rapacious as ever, are unlikely to give way, so you'd have to take them to London Tribunals and risk the full PCN penalty. And as de minimis is subjective, (no definition of it, you just know it when you see it), you could well lose.