Spotted...

What kind of helmet is it?

If it's a skate style lid it might be ok- I've no experience of those, but know that they can take a couple of knocks.

A standard road/MTB helmet however is beyond use after one hit.

There may be no visible damage, but they are designed to deform and absorb the impact force once and once only.

The last couple of times I have had to replace my lid I sent it back to Specialised as part of their crash replacement program (theoretically they cut the old helmet up and see how it performed in the crunch) and got my new helmet for half price.

Thank you for the advice, very much appreciated!
Hmm, so new helmet it is then. It's a standard road helmet, think it's a Kask, italian brand. Will look into if they do the same thing as Specialized, if they do it would make me very happy. If not, is it wrong to ask the guy who hit me to at least pay for half of it? Had it not been for him I wouldn't have crashed...?