Dual Enigma

When schoolboy Jason Cutler is killed late one night in a hit-and-run, the police are puzzled by the presence of a letter in his locker, to which is attached a five-pound note. They are unable to identify its author, nor can they ascertain why the boy was out so late at night. Solicitor Rosa Epton has followed the case with interest, but when a client she has recently defended on a drugs charge turns up dead in the grounds of the school, her involvement becomes professional.
It seems unlikely that the two cases are connected, but why does Jason's brother deny having seen him in the days leading up to his death? And who was the girl seated opposite Rosa on the train and behaving erratically on the day news of the accident came out?