Solid advice on the IAS, thanks FTMDave. So what's your advice on the invoice? Just ignore it?

Hi forum hive mind. I seem to have bad luck with these quite often! The latest one that I would love some advice on is as follow: Christmas eve, travelling to see family and pull in at a service station on the A64 near York. There is a drive-through Starbucks that we swing through (it's deserted, therefore seems like it'll be the quickest). I have myself, and my wife in the front, and my 6 yr old in the back. We place our orders which unsurprisingly consist of a couple of hot drinks among other things. As I pull out of the drive-through lane, I turned onto the roadway that serves as access to the Starbucks (and only the Starbucks) and the lid of the large hot coffee that either myself or my wife had just placed in the centre console pops open and spills a bit. I checked that there wasn't someone behind me (nope... the place was deserted still) and pulled over immediately to sort the spillage. I had my young daughter in the car and I certainly didn't fancy risking burning myself on the next corner! I stopped for 30 seconds AT MOST, on what later transpired to be a double yellow line that was monitored by a camera (and yes, there are small signs up around the property... they are too small to read by driving past, so not sure exactly what they say. I can only assume they say what they need to!). a week or so later, hello PCN (£130 reduced to £60 if I'm a good boy and pay immediately). My wife filled in an appeal as clearly, this is anything but fair. A number of weeks later I hadn't received anything in return, so I wrote a letter to the enforcement company (UKCPS) saying that as they hadn't responded they were in breach of their own terms, I was considering the matter closed. A little over a week later I got another letter from UKCPS saying that they had written to me following the appeal (they enclosed a 'copy') and they had declined the appeal as the PCN was valid. I have tried to lodge an appeal with the IAS, however, due to the delay in the appeal response coming through etc the IAS now think I am beyond the 21 days (which I am not!). Does anyone have any advice on how to deal with this? I have contacted Starbucks who have told me that there is a different company operating the outlet and they will send my details to see if there is anything they can do. The issue is that route is taking time and I'm concerned that any day now UKCPS will escalate the issue to a debt collection company and then my chances of getting anything resolved are screwed. Many thanks in advance!