Too Soon to Die by Henry Wade

Colonel Jerrod has just six months to live, but he needs a year if he is to save Brackton, the family estate, from crippling death duties. Then his ambitious son, Grant, has an idea, one that develops into a complicated fraud that, he hopes, will safeguard Brackton for future generations. But there is a boating accident, in which Colonel Jerrod is believed drowned, and a visit at Brackton from the Inland Revenue to clear up some routine questions... Before long, Chief Inspector Poole finds himself drawn into the investigation with questions of his own.