Who knows whether the helmet would have made a difference had I hit my head on the actual helmet, it would just be speculation, though I’m certainly not going to stop wearing it.
The actual impact was taken wholey by my chin, my hands were still on the (flat) bars when I hit the ground, as shown by the grazes going across my brake levers and over the top of my hands/forearms. I suspect I was lucky not to break my jaw/a lot of teeth, 2 stitches in my chin and a chipped tooth was the extent of the damage, given I was probably travelling about 20mph. The site of the bleed (and the hole they drilled) ended up being about two inches above my left ear though, so had the hit been to the helmet it potentially could have dissipated the force enough to stop the brain bleed.
The condition ended up being a chronic subdural haematoma, which developed after the inital CT scan I had a few hours after the accident (which showed nothing out of the ordinary). Most worrying for me was that the symptoms took so long to develop (over three months!) and could easily have been ignored until more permanent/life changing damage had occurred. There is probably a lesson to be learnt about getting a check up after any serious head injury!
As it is I think I got very lucky, aided by bothering to get the symptoms checked at A&E and most likely my age/fitness making recovery more straight forward.
Edit to add: The bleed was not definitively attributed to the accident, but they were almost certain that it was, especially given the unlikeness of it being spontaneous/unrelated given my age.