As I understand it:
2)Subject to the provisions of this section, where—
(a)a penalty charge notice has been cancelled under paragraph 2 of Schedule 1 to this Act;[ authority accept representations from RK to the effect that the vehicle was on hire],
then this time limit for serving a new PCN applies:
the borough council or Transport for London, as the case may be, may not serve a fresh penalty charge notice after the expiry of the period of 28 days from the date of the cancellation of the penalty charge notice
In short, 28 from date the 'garage's' PCN was cancelled, not 6 months.
The extended period here suggests that the OP's PCN is invalid by virtue of being served late.