Thanks for the information so far.
In the UK our mains electricity is 240 volt, so the US mains info isn't relevant to my setup, but I'm sure it will be useful for other readers.
Transforming (inverting or whatever) all the way up to 240 volts then taking it back down sounds like it would be energy inefficient and more than is needed to just do the job I need.
Best bet (for us) so far sounds like the Maplins gizmo mentioned by Dave Bryant. We'll probably go for that option and test the laptop on our caravan's spare battery to see what sort of performance and duration we can get doing the heavy load tasks such as playing DVDs. I know that caravan batteries are more heavy duty than a standard car job, but don't know by how much.
I'll report back with any useful additional data.
Quack!
GtD.