Graham,
I regularly use my laptop (a Dell) with a 19.5VDC requirement and I ended up getting a DC-DC converter. Jaycar sell them and you select the voltage on the front and it comes with a standard 12V "cigarette lighter" plug and about 5 DC plugs which you pick the one that fits your laptop at the other end.
If you're into building electronics, here is an article from this month's Silicon Chip.
http://www.siliconchip.com.au/cms/A_112454/article.html
The Jaycar one is here :
http://www.jaycar.com.au/productView...T&SUBCATID=922
Both work very well so you only have to worry about your 12V battery capacity and charging.
Good luck!
Darrin...