This is advance booking, so make sure you know what train you want first!
I'm off to London for New Years, and was looking at train times over Xmas. The Virgin website was seriously slow, so I called them up on 08457 222 333 (if you start talking when the "Voice Activated" system starts talking, then it'll say it doesn't understand you and will put you straight through to an agent). I gave them the dates and times I wanted to travel (Stoke-Euston 31/12 and Euston-Stoke 03/01). I was told that by booking First Class seats in advance, it cost £17 outbound, and £18 return leg. I pay £34 for an open return with a 16-25 railcard, so an extra £1 for 1st Class seemed worth it.
I hope that this is a decent offer, and not something that runs all year round.