Thanks everyone - I actually feel even worse this morning when I realise what could have happened.
It seems they were at Hawes on the way to the Lakes - enthusiastic riding, round a bend and straight into a lorry coming the other way which had straddled the white line in order to fit round. Harl dropped his bike luckily but the other 3 involved were fairly badly hurt - one of them has broken both legs in more than one place and his arm in more than one place, the muscle was also bulging out of his arm, pillion was thrown through barbed wire into a field, she has a broken leg, not sure about the other guy but he also has at least a broken leg, my son is the only one involved who stood up and walked clear as they were in front of him. The problem we have now is what to do with the bike - police have impounded it pending an investigation but then what will they do with it? Obviously he is now worried they will charge him for keeping it but he wants it back to break up and sell the bits - at least he'll get something back then. Oh and Bennets Bike Insurance were absolutely useless and unhelpful!