aka I just flipped myself out of the chair backwards.
I think I was a bit energetic in decoupling from Asda's wheelchair trolley when I took it back after putting my shopping in the car, and when I stopped my wheels the chair pivoted on them. I flung myself forward, but couldn't quite get the momentum to stop it, but it did make it a fairly slow motion flip. And curling forward also meant my head missed slamming backwards into Asda's plate glass windows by about an inch.
Of course people came running, and I had to stop them from trying to pick me up, which wasn't helped by the fact they were a bunch of foreign schoolkids and didn't understand what I was saying!
Amazingly the two bottles of spirits in my back-bag also survived, though the two loaves and sadly the box of Magnum icecreams may never be the same again. (Doubly amazing for the spirits as there is a very prominent projecting bar across the back of the chair)
I'm okay, just literally a bit rattled. My neck hurts a bit, but it wasn't exactly happy before I left the house.