IMO, the '28-day period' has been misstated.
This occurs in 2 stages. The first is clearly mandatory, the second less prescribed but arguably of equal weight.
The first reference is on page 1 and is the warning regarding the implications of neither making representations[in accordance with reg 5 of the Appeals regs] nor paying the penalty within the 28-day period:
(f)that if after the last day of the period referred to in sub-paragraph (d)—
(i)no representations have been made in accordance with regulation 5 of the 2022 Appeals Regulations, and
(ii)the penalty charge has not been paid,
the enforcement authority may increase the penalty charge by the amount of any applicable surcharge and take steps to enforce payment of the charge as so increased,
(d) that the penalty charge must be paid within the period of 28 days beginning with the date on which the alleged contravention occurred,
The regs don't prescribe form of language to be used to convey (f)(i) and in this case the authority have chosen:
'..representations which are made after the end of this 28 day period['within 28 days of the date on which this notice was served'] may be disregarded.'
And IMO it's wrong.
There's only one '28-day period' and it's 28 days beginning on the day of service, not within 28 days because the latter adds a day which in this case could leave the keeper exposed because they'd risk becoming liable to a surcharge penalty.
Some thoughts for discussion or destined for the long grass!