Innes, Michael - Carson's Conspiracy

Carson, a higher-level businessman, is in financial trouble, so he arranges the kidnapping and ransom of a non-existent son (to liquidate his cash in order to flee to Brazil). Appleby gets involved (this takes place, again, in the neighborhood of Long Dream Manor), and this time he screws up, saying at the end to Tommy Pride: "Mycroft is retired." One can say that Sir John has been creaking at the joints for the last few books. Still, it's a good plot and has a nice surprise ending.

Wyatt James