How to obtain third party website security pin you created but have forgotten

Pardon me for saying so, but it seems to me that "third party e-commerce website" was intentionally designed to defraud you of £14000 while holding your funds for ransom.

You may in fact using the correct PIN.

Call my cynical, but that's my opinion. I have no idea what "Plantar" is or what it does but coercing users into parting with £14000 for what ought to be a simple task sounds like a pretty solid e-commerce business model.

If this is a pin created by the 3rd party company, then there's prodigal nothing form the iPhone side you can do. The Pin is unlikely to have been saved by the iPhone if it was created using an external process to the iPhone through a website.

The iPhone may save users and passwords in the Passwords app for said website, but anything else, you be your responsibility to keep in a safe accessible place.

You will need to follow whatever steps they are telling you to follow to change it.

Why do you say they are impossible?