Who should people contact if there is a problem with their payment?

The TfL Website will contain information on likely reasons for rejection of a payment and who best to contact in each instance, but in practice they recognise that customers will choose themselves who to contact.  The TfL contact centre is initially planning to try to deal with as many issues as they can directly, even if it is theoretically an issue for a bank.  TfL have also been in close contact with the card issuers and have sought to establish that they will take a similar approach.