Well, luckily the case with the disks is still in my drawer, so I just checked and it was disk 1 which bust, it's odd that it happened to you too. The wierd thing is that it seems to have cracked from the centre to about halfway across its radius, rather than all the way to the edge, as you'd expect a disk to do if it broke.
I've never had any other disk do that, maybe there was a shaky batch if yours also broke, as you are in the UK too? And it just broke on its own, I opened the case to play it one day, and there it was, busted. That was in fact why I got DW via download rather than buying it on DVD!

Chock