yeah i have a multimeter, its reding 12 volts from the battery, but someone told me a battery reads 12 volts even if its dead anyway! ive just tested the inverter in my car by plugging it into the cigarette lighter, it works no problem. I then tried hooking it back upto the van system and as before it powers up ok from the leisure battery but no power on the lights or sockets. It seems to me that the battery may be not giving enogh power for the lights but enough power to boot up the inverter, i just dont know!
Do you have nothing to check the state of charge of your battery?
Buy a cheap multimeter from maplins or ebay, only about a fiver, and absolutely necessary for a solar system.