I wondered if anyone was able to offer some advice on this;

I have booked a return rewards flight from London to Las Vegas with Virgin in Upper Class. But now, a few weeks later, my wife has decided she wants to come with me. (That's not the problem, by the way! )

I don't have any more Flying Club miles available, so I'm thinking about buying her an economy ticket on the same flights. What's the chance of getting her bumped from Economy to a spare seat in the UC cabin if I nicely ask a stewardess after take-off? Has anyone ever had a similar situation, and if so what was the outcome?