Well a couple of hours ago i received a reply from Horizon Parking (after around 4 days)...
Dear Appellant,
Parking Charge Notice: XYZZY-PLUGH
Thank you for your recent correspondence regarding the above Parking Charge Notice, which we have reviewed.
I can confirm the Parking Charge Notice has now been cancelled and no further action will be taken in this matter.
Please note that once the Parking Charge Notice is cancelled the contact details are removed from the Parking Charge Notice and will not be retained for any purpose or shared with third parties. We are, however, required to retain images and appeal correspondence for a period of 2 years by DVLA and the BPA and are not therefore able to delete these from our system. You can review our privacy policy at https://horizonparking.co.uk/privacy-policy/ or you can request that a copy is sent to you in the post by contacting us at the address below, if you have any queries regarding your data.
Yours sincerely,
Horizon Parking
...so many thanks to b789 and The Rookie for their replies to this thread, and to the others around the forum who suggested various PoFA appeal wordings.
I must admit that i am surprised how quickly Horizon backed down, especially as they must have spent a few quid requesting my details from the DVLA plus the cost of postage and printing. Gotta give them credit for not trying to drag things out!
Time for a celebratory beer