First sourdough bake was a success!
Thanks to @MrAndrewD for the starter, and @elaterite for the tip on feeding it with rye (it helped, my first feed didn't do much, but the next one, with 25% rye, went bananas).
It's too early to know if the ciabatta tastes good yet, but I'm happy with how it looks.
But the bagels are great, if slightly beery-flavoured.