After 12 frustrating attempts online with the "loading circle of death" followed by a crash and the system telling me I don't have any ticket allocation remaining, then rinse and repeat, I decided to call customer services.
After an hour and a half on hold I eventually got through and spoke to a guy called Paul who sorted the issue out straight away in a most polite and professional manner. He was absolutely superb, and I felt it only fair to come on here and praise him after all of the bad press.
But the IT set up for buying tickets is an absolute fucking shambles, and it is not fit for purpose in so many different ways. They need to rip it up and start again.