How did you shear the bolt?? I think they way M0rk describes is the best. Shearing of a bolt is very uncommon. It is usually caused by hydrogen embrittlement of the metal. I.e. Hydrogen gets trapped in the metal during plating causing it to be weaker at that point. Normally spoken this is counteracted by extra processing after the plating.
Are you running original bolts or aftermarket ones? If original take it back to the dealer and demand a new one and get them to send it to Seat UK. If one goes then usually a batch is affected.
Not a good thing