My son has gone on holiday and has just phoned to say that when he pulled into hotel car park he noticed water dripping from front of car - no indication of overheating on the dash. He found a sheared plastic screw on the top of the radiator next to the filler cap, which I believe is a vent screw. Any idea what would cause this to shear and any tips to get him home safely, as this is Bank Holiday weekend? Any advice much appreciated.