>As much demand as there will be for the Potter play, the current theatre size would be a real negative. It is, after all, a drama, and people aren't going to be happy paying a lot of money and not having any chance of seeing the faces. That is more forgivable with a musical.<
It is playing London's Palace Theatre, which usually houses big musicals. That's a 1400 seat theatre, with three balconies, no less, and there's not a seat to be had.

Begin at the beginning and go on till you come to the end: then stop.