Power Switch – £7.27
This is a manual switch to change between external power and battery power. One of the inputs comes from the external socket and another comes from the power inverter, the output goes to the consumer unit.
(http://en.wikipedia.org/wiki/Utilization_categories)
This has been in and working for ages now. My next step is to wire the 12V through this as well so that a single switch flicks over both mains and 12V!