Death on the Cherwell - Mavis Doriel Hay

I´m probably wrong, but I have a hunch what the motive for the murder might be:

 

Pamela is Mrs. Dennings daughter and the murderer is Pamela´s biological father. 

(show spoiler)

 

And I have a hunch, who the murderer might be:

 

Mr. Mort, who had muddy feet when he arrived at the appointment with Sally. The Scotland Yard didn´t pay much attention to his appearence, though, so either he is a bad police detective or I´m wrong again.

(show spoiler)

 

I´m really not enjoying this book at all. But there are only 80 pages left and I should be able to finish them today.