Help requested tracing the literary executor of the late Professor Roland K. Harrison

In order to obtain permission to reproduce a couple of his articles I am trying to find out who currently holds the rights to R.K. Harrison's works when these are not held by specific publishers. In most cases this would be the author's next of kin as it forms part of the deceased person's estate. Professor Harrison is most famous for his excellent Introduction to the Old Testament. I have tried all my usual avenues of enquiry but have drawn a blank and my e-mail to his last place of work has gone unanswered. Does anyone have any ideas of how I might proceed? Feel free to use Blogger's "comments" feature to respond - I won't allow confidential information to be published on the blog.