In that same book, Amanda stalking said serial killer in order to get a good description of him to the police, and hopefully save her friend who the killer is after.
Amanda figuring out who the murderer is in "More Work for the Undertaker" while only hearing about the case from the papers and brief letters from Campion.
This description of Mr. Campion in the final story Margery wrote:
In his own apologetic way Mr. Campion was a celebrated figure. In his time he had performed a number of services for a great many causes. He was a negotiator and an unraveler of knots, and there were still people who suspected, because of his wartime activities, that he had a cloak and a dagger somewhere concealed. Those who disliked him complained that he seemed negligible until it was just too late.