the rest of our free kit arrived from HP this week including the extended battery.  It's a wedge shaped block that sits under the machine, is very light and you can charge it independantly. 

Having charged it up, I tried it out today:

battery

 

sorry..?? 39 hours????? blimey.  then half an hour later..

battery2

 

what a truly incredible piece of kit!  Not only does it have enough juice to run all week, it even creates energy :-)

As Rob Gray said earlier, perhaps the EU might be interested as a solution for the working time directive - you charge it up at the weekend and keep working until your laptop dies, ensuring you stay within the safe working hours limits.