Hi Johnno.
First of all, that's a fairly blurry pic, so it's probably gonna be hard for someone to make a definitive judgement call.
There's a chance it could have been a white-tail spider. These leave a characteristic bite which is very slow to heal and very destructive to the tissue surrounding the area of the bite.
My brother was bitten by a white-tail and it took ages to clear up. It left quite a hollow from all the tissue which was lost.
But yours could also be a large number of other things.
I'd go see a GP if it gets any worse. Otherwise go to a chemist and seek advice.
Good luck.