What’s the best way to get an upgrade to first tomorrow (Saturday 18th April) between Plymouth and Bristol on CrossCountry? Seatfrog are saying £33 for an upgrade but https://www.brfares.com/!faredetail?orig=PLY&dest=BRI&expert=on&flow=2&multi=0&fare=0&rte=24&tkt=XCU suggests it should only be £15. Am I missing something?