Thank you. Sleeping on it, and writing this post, has definitely bought things back into perspective: we can all have off days.
It had been a while since I'd had this sort of negative experience, and what I would say is it's re-reminded me to make sure my hiring in the future caters for this sort of thing. Particularly for less experienced devs.
It happens. It's the equivalent of writer's block and the fear of the blank page. Please just don't read too much into individual events.