I want to travel a section of a journey from Darlington to the Metrocentre in first class (the Durham to Newcastle leg), and am breaking my journey at Durham. At the moment, I only have an off-peak single from Darlington to Durham. What's the best way of excessing this ticket to cover first class from Durham to Newcastle, or am I better off just buying a new (flexible) ticket to cover this leg? Much though I don't like them I can use a northern advance from Newcastle to the Metrocentre, currently I've got £21.30 as the cheapest including the ticket I already hold. Thanks in advance.