Double Screen Inverter Problems

My Advent 7108 goes dark after a couple of seconds, but if I look really carefully I can still see the graphics, cursor moving etc and the atatched monitor works fine. If I toggle between the monitor and the laptop screen the picture comes back for a couple of seconds. I see a high voltage on the output of the Darfon inverter for a couple of seconds (not not long enough to get a stable reading on my DVM) then zero volts.
Classic inverter problem you may say, but exactly the same thing happens with the replacement inverter. Have I just been unlucky and the repalcmeent unit (sold as tested and working) has exactly the same issue as the original?
I don’t see how it can be the screen since the inverter cannot sustain an output voltage with the screen unplugged. So is there any way this could be something to do with the motherboard? I figure it has to be a hardware issue, since exactly the same thing happens when I first boot, i.e. before windows is operating 'normally'.
Am I being unreasonable to expect the supplier to send a replacement or refund?
Thanks in advance
Robert