I am a little surprised about your comment about rubbish battery life. I am getting 5 hours out of my Medion Atoya E1210, which I find very good (after my now sold large laptop struggled to give me one hour). Do you turn off unnecessary functions, such as wireless receiver, webcam, etc, when you are mobile? Have you set power saving for efficiency?
I bought a 12volt converter from eBay, as you have, but frankly have never been brave enough to use it! Scared to death in case I get polarity wrong, and burn out my precious little netbook. Personally, I am waiting for Aldi to start selling their 240 volt inverters again for around £18 (if my memory serves me well) as I feel I trust that more to give me continuous use when in a vehicle.