Just curious, but if you drive with the radio on at any time you may not hear a stone hitting something, probably less so than a stone hitting the wheel arch on a gravel road.
Also in you comment you said " I have no proof of this but when I got home to Hitchin there was a nasty chip on the bonnet, which most definitely wasn't there before I dropped it off and seeing as you have to drive so slowly in Letchworth and Hitchin I would of heard whatever hit the car on my way home."
But didn't you say your wife drive it home, were you a passenger? Or did you wife pick the car up from the dealer?
Shame about the stone chip though.