Pete,
found this thread from 12 months ago which confirms my thoughts on switch function. http://www.caravantalk.org.uk/topic/27341-appliances-in-elddis-count-1993/
The poster at that time asked for a copy of manual but did not appear to get one.
Also found posted by Gary from ARCSystems, who is recognised as an expert on van heating systems, this info re Primus
"I stay clear of them due to a very nice man in the West Country who knows far more than I, try Bernard Darvill on 01460 62695"
With system filled with water, Mains Electricity on, Battery in place. Press button with gas flame and burner should light also green light above will light. The two with Lightning are the electric heating and one with two flashes will give 2KW heat the one with one flash 1KW heat and both together 3KW heat.