Their reply is ignorant.
The law:
..
But no penalty charge may be imposed under paragraph (1) in relation to a parking contravention where—
(a)the vehicle is stationary in a designated parking place and is left beyond the permitted parking period, and
(b)the period for which it is left beyond the permitted parking period does not exceed 10 minutes.
But they did impose a penalty as at the time of the contravention.
No-one cares a hoot about time taken to serve the PCN.
IMO, carry on.