12 volt running - am I missing the obvious?

We have recently bought our first motorhome - a lovely condition, one previous owner 2001 Autotrail Mohican on a Mercedes 316 base with only a few miles on the clock.  We have had a full habitation service and a new leisure battery fitted.  

Is someone able to offer any advice with regards to using the 12 volt system?  To date, we have only used the lights, etc., using an electric hook-up to the house as we haven't managed to get away yet and it's off for its MOT in the morning.  Today, however, I disconnected the 240 volt supply and assumed that the van would automatically switch to 12 volts running.  However, nothing shows up on the control panel, no lights work etc.  

Is there something blindingly obvious that I am missing in terms of switching from 240 volts to 12 volt operations? I have read and re-read the manual.  I have also re-connected the mains and the control panel works fine. The battery voltage registers, the pump works, etc., with the 240 volts connected.  I have 'rebooted' the control panel in accordance with the manual when connected to the mains but still nothing showing on the 12 volt front on the control panel and none of the 12 volt lights working etc.

Thank you