You have to file a defence or Acknowledgement of Service by 6 July.
If you file AoS you have to file a defence by 20 July.
Please post the original PCN to see the basis for the defence. You say that it was issued too late; what are the relevant dates?
If the PCN is issued on 20/1/25 for an alleged contravention on 8/1/25 then it will be deemed to be delivered on 22/1/25, 14 days after. Any later issue date will make it too late for the requirements of PoFA 2012. Smart has form for lying about only needing to issue the PCN within 14 days of receiving the details of the registered keeper, rather than within 14 days of the parking event.
(4)The notice must be given by
(a)handing it to the keeper, or leaving it at a current address for service for the keeper, within the relevant period; or
(b)sending it by post to a current address for service for the keeper so that it is delivered to that address within the relevant period.
(5)The relevant period for the purposes of sub-paragraph (4) is the period of 14 days beginning with the day after that on which the specified period of parking ended.
(6)A notice sent by post is to be presumed, unless the contrary is proved, to have been delivered (and so given for the purposes of sub-paragraph (4)) on the second working day after the day on which it is posted; and for this purpose working day means any day other than a Saturday, Sunday or a public holiday in England and Wales.
If this is the McDonalds on Mondial Way then we have previously established it to be outside the area of statutory control by Heathrow Airport.
Presumably you did not appeal to POPLA, who may well have cancelled this for you?