Thanks Rajah
Guess what? I think I've got my head around it all now. I'll repeat it back to you so you can check:<ol>
<li>Don't use a 12v power supply to run a laptop requiring 18v!
<li>Avoid running other accessories from the same power supply as the scope!
<li>A cigarette lighter adaptor is a good thing. It will run the scope for ages without flattening the car battery!
<li>The Supercheap models will be fine, but...
<li>Bigger is Better!</ol>
I think I'll just go for the 12AH model and see how it goes. If I find I need it I might go with the Jayco for the laptop as well down the track.
Thanks for all your help!