Date of contravention: 16 March;
Date PCN issued: 18 April, deemed served Mon. 22 Apr.
Elapsed period contravention-service = 37 days.
6Limitation on service of penalty charge notice
(1)Subject to the provisions of this section, no penalty charge notice may be served under this Act after the expiry of the period of 28 days beginning with the date on which the alleged contravention or failure to comply occurred.
IMO, on the face of it the PCN is void by virtue of being served outside the permitted period.
OP, is there more to this e.g. vehicle is hired/leased etc?