You put them to strict proof of the date of actual sending based on the requirements of section 8.1.2(e) Note 2 of the PPSCoP which states:
The parking operator must ensure that a notice informs the recipient: that if the recipient appeals within 28 days of receiving the parking charge, the right to pay at the rate applicable when the appeal was made must stand for a further 14 days from the date (subject to 8.1.2d) they receive notification that their appeal has been rejected;
NOTE 1: Where the 14 days to pay was at the reduced rate a further 14 days should be offered to make payment at the full rate.
NOTE 2: A notice sent by post is to be presumed, unless the contrary is proved, to have been delivered 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. Therefore, parking operators must retain a record of the date of posting of a notice, not simply of that notice having been generated (e.g. the date that any third-party Mail Consolidator actually put it in the postal system.)
