The capacity for the theatre is 300, though I think Roadie 42 said it was 250 in his blog. Either way, there were a whole load of tickets not available to the public: 70-120 seats depending on which number one believes.
I was told that there was a massive BBC waiting list for this gig earlier in the day, about 45 minutes before Lost!, myself and another fan were given tickets for that night. There were at least 2 others who came down and ended up getting into the show as well.
I do know there were a fair amount of seats reserved for Parlophone and obviously Beeb staff. Regardless, the crowd was definitely into it. It was a brilliant night.