But what a hassle. First I had to repeatedly fill out the form, because the site failed to finish the booking process. At first I had seats in row B which I lost. Then I had forgotten the security code for my credit card and had to call the company. It already was half an hour after ticket selling start. Now the site said, no tickets available at all (for the best category in the stalls, I could have gotten some far behind).
But I kept refreshing and finally was rewarded with tickets in row D in middle block! And this time everything went smoothly. Phew. My guess is, that those were claimed by somebody but then also lost due to failed booking, and so they were available again.
See you all in Cardiff! Good luck to those still trying!