EVAP codes (but not P0443) can be caused on Mustangs by ill-fitting filler-caps, but not on our cars, I believe.
Since you bought the car I guess on or about 14 September (??), which is when they stopped listing it, you should go back and point the dealer to the word "guarantee".
Failing that, get him in any case to read the error code causing it and tell you what is wrong according to the code, and how he intends to fix it.
Reason is that in the worse case of an EVAP error code such as P0433, a new ECU or ECU repair could be necessary. That costs money. Don't want to be 'doom-and-despondency', but now is the time to act, I think.
... of course the code may not be that at all, and it might just be an O2 sensor code, say . Even then, get the dealer to prove it, ... and fix it. You can't really sell a car with an EML up without explanation? Not sure about the UK, but here that is an APK (MOT) failure.