Thanks to some help from the tomcat-users mailing list the solution to my problem was as follows:

Stop all Tomcat instances/service. Open the "tomcat monitor" application and open the "configure" pop-up/window. Or open the "Configure Tomcat" application. On this window there is a "Java" tab, go there.

Within the Java tab there is a text box for Java Options, in this text box append the following:
-Djava.security.manager
-Djava.security.policy=<path_to_security_policy_file>

The standard Tomcat policy is located in Tomcat's conf/catalina.policy file, but you're free to specify whatever location you are using.

Cheers