Just thought I would give my thoughts on the battery extender from shirtpocket. For aound 8gbp (batteries not included I doubled my battery life for my 5600 and no need to worry about "flipflopping" and other scary tales from people who build their own battery packs.
Only problem is the cable is so short, it's really hard to use on the move or even at your desk since its about 25 cm long. I got around this problem by making myself a battery extender holder out of velcro and black elastic, prbly used to make tracksuit bottoms for very fat people (which i strapped to my wrist, and since i wear a jacket no one can see it), also i'm of to my local maplins to see if i can get a cable extender
All in all im very happy with it