Today I decided to do some further investigating into the heating issue, specifically the ductwork around the furnace area. I decided on the hi-tech approach, and stuck my digital camera into the ductwork. The results were a little surprising. They are shown below in a plan sketch of the ducting.
So it now seems that other than taking the base off the rig and mosifying the ductwork, the best solution is to:
1> Put adjustable registers into the living area and close them down at night,
2> Clean up the ductwork in the storage area to give a smoother airflow.