You need to pay online using the app or by phone quoting the car park code number. As I understand it is like Bristol Parkway and there are two codes. If you pay using the publicly available code it is very expensive £30. However once you are on the platform there is a dot matrix sign with another code which if you use that you get to pay the lower amount £6.70. It is to try and ensure the car park is for rail users only.
Aha, great stuff, thanks for the confirmation.
I don't think Bristol parkway has a separate code now, it's just one high price!!
I remember when the app came out it was billed as the cheapest way to pay, but not now - they even add a 20p admin fee so it costs more than using the ticket machine!! But that's not a discussion for here!