RangerSteve:

Great-Great-Grandfather Radford and a Mr. Boxer own a brewery about 1850.
Boxer has to leave town because he has gotten a woman in trouble.
GGGF Radford didn't pay Boxer for his share in the business.
Unknown to all Victor Preece and his Mother are descendents of Boxer.
Mrs. Preece asks lawyer Nelson if she has a right to part of the brewery.
Lawyer Nelson says "Not Likely".
Because Victor Preece was cheated of his inheritance, he murders the Radford sons.
A third murder takes place, lawyer Nelson is found dead.
Lawyer Nelson had used Mrs. Preece' information (skeleton in the closet) to blackmail the Radford parents.
Morse determines that parent Mrs. Radford is the person who murdered lawyer Nelson.

As Q states Mrs. Radford was found out by her "Upper Class" use of English.
The sub-plots, suspense, sleuthing and character quirks are missing but this is an overview
as I remember the episode.