Auxiliary Heater Help

[oracle]

2008 Cali 4 Motion.

My Auxiliary Heater is very temperamental, often not firing up just blowing cold before shutting down. It is a little more reliable straight after a run i.e when we have max charge in the battery.
My local dealer seems to be stuck. They have added an earth just in case, tested glow plug and heater element which are fine.

If I read my owners handbook correctly the Aux heater is powered off the battery in the wardrobe but they seem convinced its the one under the passenger seat which is new.

Does anybody know which battery it is or have any other ideas what the problem might be?

Thanks Steve