Don't all rush at once. The catch - of course there's one - is that they're for Jonathan Miller's creaking production of The Mikado, back for its 953rd run.
The offer, which also covers dress circle and upper circle seats, is bookable via SeeTickets. You don't get to pick where you sit, but you are told exactly what seat numbers you've been allocated once you select the quantity.
Thanks to reader Richard for spotting.