Hi guys, thanks for the replies.
I decided to get a doctor to check it out, just to know whether it's anything to worry about or not.
What she basically said is that it wasn't dangerous. I had obviously received a pretty heavy knock. However as far as I can recall, the dent is in some kind of cartilage running along the shin bone itself.
What she said was that the shin bone is extremely hard, so it would really take a lot to do any real damage to it. And if this was the case, I definitely wouldn't be able to put any weight on it at all and would pretty much know instantly that it was bad.
The indent would maybe go away, but not for sure, so all you guys with indents all over arms or whatever needn't worry