Needs to be the Oracle JDK. Download the .bin file and make it executable. Create appropriate directory, change to it and run the .bin file.
update-alternatives --install "/usr/bin/java" java" "/usr/lib/jvm/jdk1.6.0_31/bin/java" 1
Creates link for java in /usr/bin/
update-alternatives --set java "/usr/local/bin/oracle-java-default/bin/java"
Not needed for a clean install - there are no other versions of java on the system.
Then in /etc/environment
JAVA_HOME="/usr/lib/jvm/jdk1.6.0_31"
Gave up at this point and decided to give DAViCal another go.