I've been trying to track down what might have bitten me in NW3 four days ago -- it looked shiny and black like a beetle as well, though the size (what little I saw of it as I reacted instinctively to brush it off) was much larger than what you are describing. I *think* that what bit me was 1/2 to 2/3rds of an inch long.
Four days later I still have an irritated, itchy red patch about the size of a 5p coin on my hand with a small red dot (presumably a scab/scar) in the centre. It hasn't improved, but it also hasn't become worse. At some points of the day the centre seems more white than red, but then it gradually goes a light red again.
I wouldn't be concerned except for the duration and that I have no earthly idea what did it or where it came from (I had just been food shopping and am wondering if it was a hitchhiker from some other part of the world.).