Wow! I am genuinley speachless at all that bollcks!
With regards to wanting a legal battle, hope your got some spare cash lying around. Solictors cost will be alot for a good one who will take the case and someone who knows the sales of goods acts etc, so bare that in mind if you want to battle them, (might be able to get a recommendation for you consider that route). Constant emails, and daily updates pestering, asking for managers to contact you by a certain time, and if not you want the managers manager etc etc going up the ladder and see where it gets you. If your willing to battle, someone with the power to resolve it will get hold of it and either be fed up and give in, or take you all the way. You have a strong case of issues prior to this supposed "stone failure" (looks alot like a spanner has come off the head and hit it imo), if your committed to it, then best of luck.
Failing that, trip up to Renault HQ with a stack of images paperwork a solid well planned argument and collar the Head of Cusomter Service, or even Renault UK MD (Ken Ramierz) or Renault Retails Group MD (Michal Kalina) - accoding to google.
I would like to hear a detailed anaylsis of how the can say that a stone has managed to get past all the protection and covers to causes a precicse dent. Would it not possibly get caught in the belt and dragged round the pulley, leaving some scoring/ scratch marks? If they are saying its a stone, surely some reasonable justification of how it "might" of happened is reasonable for you to ask, rather that giving that as an excuse.
Gorms issue was paint. Went in several times and was never right, in the end solicators letters and finally it was rejected and Renault accepeted it.
Could try the Social Media way also. Spreadding the word etc. Twitter Facebook.
Tweet Renault UK directly complaining, saying why havent I recieved call backs, straight into the public eye.
All in all, best of luck!
Could always try and watch channel 4 "supercomplainers" and see if you can get and tips haha!