Sherlock Dan has solved the Case of the Crumbly Bread.
And like most cases, the reason is pure idiocy on my behalf. I thought I put in 340ml of water. But, the measuring cup is 200ml and I put in 200ml and then another 40ml thinking I'd already had 300ml in there.
So did another one last night with the correct amount of water and the result was pure, heavenly breadiness worthy of Jesus himself.
Maths. Every. Single. Time.