Well basically they put another date on and, *at the moment*, the Roundhouse box office website (the venue's) will let you buy standing tickets for the Friday show OR standing/seated tickets for the Saturday show. I bought mine at lunch time but they're still listed as available...
http://www.roundhouse.org.uk/whats-o...on=performance
Give it a whirl.
EDIT: Friday is OFFICIALLY SOLD OUT.
EDIT #2: I got my order confirmation for Saturday's tickets through okay; £52 for two. Apparently I'll have them in a week.