By Michael Holden and Humza Jilani LONDON (Reuters) -Prince William has told how walking behind his grandmother’s coffin brought back challenging memories, alluding to the death of his mother, as mourners from all walks of life queued for hours on Thursday to file past the late monarch’s coffin. The new heir to the throne told well-wishers that the military procession had “brought back a few memories”, a nod to the day 25 years ago when, as a boy of 15, he had followed the casket of his mother Princess Diana to her funeral. The death of Queen Elizabeth, Britain’s longest-reigning monarch, has …