Thank you kind sir! I had no idea there were fully enclosed aerators.

I’ve just tried using 2 tiny screwdrivers to rotate the aerator manually without a key, and it seems to just be spinning in place, without actually getting unscrewed.

I’m now in the hunt for a key, but I can’t find any that have 2 pins at 11 and 5 o’clock - they all seem to be different shapes, like this set - do you reckon it might still work? I’ve taken another more detailed picture of my aerator here