Fred Mumford's parents don't know that he is dead. That must mean that his death was never made public. Does this mean that his body is lying somewhere undiscovered? How did he die and why has no one noticed? And how are his parents going to react if it is ever found? Especially as he will have been dead for years, but they have been interacting with him during that time.
He fell off a ferry when he was supposed to be working abroad (mainland Europe somewhere). It's explained thoroughly in the book, and a bit more briefly in the series. His body is somewhere in the North Sea or the English Channel, and no one on the ferry missed him or raised an alarm, which is why he could come home without anyone realising.