Paragraph 21
If an enforcement agency has accepted an enforcement undertaking then, unless the person from whom the undertaking is accepted fails to comply with the undertaking or any part of it—
(a)
where the contravention in relation to which the enforcement undertaking was given is an offence, that person may not by reason of that contravention be convicted at any time of that offence;
(b)
the enforcement agency may not impose on that person any fixed monetary penalty, variable monetary penalty, or compliance notice in respect of that contravention.