Paul,
Assuming that HP_JAVA_HOME is a typo in your post, it should be HQ_JAVA_HOME.
If you did a full install, the agent is probably using 1.4, which is the JRE we include. Install without the JRE and set your JAVA_HOME and HQ_JAVA_HOME paths to 1.6 again.
Let me know if that works for you.
--jeremy