Chris Melas,
Thanks for your reply.
I went away and implemented everything you advised:
1. Copy callback logic from DemoTaskService.java to HumanTaskServiceServlet.java (making appropriate changes to avoid bugs)
2. Include the jbpm.usergroup.callback.properties
3. Alter persistence.xml with the advised changes
4. Include jbpm-tm.jar in WEB-INF/lib
I then redeployed the WAR file, but unfortunately, the same exception occured: "Could not connect task client".
Leaving things as is and starting the DemoTaskService, makes everything work again.
So I don't know if I have misinterpreted your instructions or perhaps there are further configuration changes that need to be done.