Thank you all for your replies.
Here is the latest.
Turns out, the place where my car was being inspected, for end of warranty, was responsible for the crack. As I had only left thier premises and it happened straight after, they had somehow contributed to the cause of crack. They will be replacing the windscreen after all at no cost to me. I'm very lucky indeed.
So tomorrow, I'm calling my insurer to add this small cost to my policy, I don't need this headache any time soon.