There's no play in the headset - the original headset was some unbranded crap, but I replaced with an FSA one and I'm confident it's secure.

In the absence of any mechanical explanation, this seems most likely to be what's going on. The issue is happening less often as I'm being more careful to simply avoid using the front brake going over bumps. I'm trying to pick my way through bumpy sections, but traffic and sheer amount of lumps means it's not always practical

I'm using first and second fingers only on the brake lever, thumb and third and fourth fingers gripping the bar. The brake lever is angled 30-40 degrees down. There's no obvious sensation that I'm involuntarily grabbing the brake harder when I hit a bump, but I can't rule it out. The geometry of that seems unlikely though - I'd have thought my palm is exerting pressure on the rear of the bar pushing my fingers away from the brake lever. It's hard to be certain though

Yeah, I wondered if it could be caused by the forks bending as I go over bumps and that changing the geometry, but surely they can't flex that much? I'm pretty confident the geometry is otherwise unchanged compared to the original forks with a person's weight on them - I mean I've not stuffed it up

The scenario tends to be limited to when I'm already braking and then I hit a bump. Braking is generally smooth and predictable provided that the road surface is smooth - i.e. I'm already braking, without any issue, and then a bump causes the brake to snatch