Hi there and welcome!  
  
I heard of this issue in the US, but our Roadster owners afaik haven't experienced this! 
  
if you have no luck with NMGB CS I suggest you to start writing letters mate! and send them to the big bosses too  
  
Of course it is a warranty issue, roof is supposed to withstand elements, nevermind cracking! 
  
sorry to hear about your problems