Once I clicked through to Ticketmaster, I had to log in with my usual Ticketmaster username and password. That I have no association with o2 clearly didn't matter by that stage.
On the phone I borrowed there was done sort of priority O2 app. Found arcade fire on there - it had defaulted to the Bristol area only to start with - and then clicked through to buy. It just opened a Ticketmaster page in a browser where I went through the transaction.
I'm also new to both Arcade Fire and Reflektor and I'm enjoying it muchly although it is being played slightly less than Chvrches currently, who were suggested by @wozza.