Just tootling around the car park.....
This view shows there to be four car parking spaces, and three long bays marked "coaches only" alongside each other, and at the back are some bays for BB holders.
https://maps.app.goo.gl/dsWJu7Szbfubj1CK9So one has to wonder how the council think the app they insist you use for PbP can possibly distinguish between the two tariffs at your chosen location. It only shows a single location, whereas it should display two, one for coaches, and one for cars. . The nearest board to you merely says you can pay by phone but gives no number to use, that is displayed on the app. The display on your app shows two numbers with no apparent difference in the vehicles that can use that number. Indeed what code do you use if you are in one of the four car bays, or the BB bays ? Clearly and rational person would select the code for where they can see on the map on the app.
The council are being their usual obdurate selves, so you really must get them in front of an adjudicator
I would suggest something on the following lines, backed up by photos of the sign nearest to your car and also the app map displaying the two numbers.
Dear Sirs
Re PCN <number> served on <dd/mm/yyyy> at Sadlers Mead Car Park
I deny responsibility for payment of the above PCN on the grounds that the contravention did not occur.
On the day in question, being new to Chippenham, I searched for parking and eventually parked in Sadlers Mead Car Park. There were spaces available to the left as I drove in, some for Blue Badge Holders, three for coaches, and four for cars. A short distance away, a sign welcomed me to the car park and told me that I could pay by phone. I therefore opened the applicable app on my phone that showed a small map of the car park, and selected the number shown nearest to my car. The number shown on the sign was different, but on the map, it was shown at the multi-storey car park. I therefore selected the number that seemed to apply to where I had actually parked and paid.
My initial challenge was rejected on the basis that I had parked in the coaches car park, yet there are no signs whatever concerning coach parking at all, anywhere in the car park, only the sign I saw. I therefore consider that I used the app correctly and paid and that the council have not given sufficient instructions to the suppliers of the app on payment arrangements where I parked where two tariffs are in operation, one for cars and one for coaches. I attach photos of the map on the app on my phone, and the sign nearest to my car.
I therefore request that the PCN be cancelled forthwith.
Yours faithfully
Just a question - were you actually parked in one of the car bays ?