.....is going to be difficult, but as I've been emailed to ask about it, I thought it might be useful to share what I know.
There are two concerts to go, on Friday 15 February, and Sunday 17 February, and both are sold out according to the Southbank website. If the ticket office receive any returns before the concert date, then technically they should offer them up for sale on the website. But in my experience, it's unlikely anything will turn up.
The next choice is to queue for returns on the day. A *lot* of people have been doing this, chasing a very small number of returned tickets. Queuing has started early, and in an unaccustomed feat of organisation, the SBC have set up chairs for those waiting. I would phone the box office first on 0871 663 2500 to check what time you need to get there to be in with a chance.
There's also the option of listening to the live relay of the concert, which is taking place in the Ballroom beneath the Royal Festival Hall. I've no idea about the sound quality of this, but judging by the fact that there are no visible microphones near the piano, I'm guessing it may not be great. The area can accommodate a large audience than it's been getting for the last couple of shows.
This might be a good place to point out that both the Southbank and Barbican have opened their 2008-9 season bookings recently, and however crazy it seems to be arranging your diary that far ahead, the most popular concerts are already selling well.