I was going to post a picture of the setup, but it consists of:
One garmin 310 with cradle
One replacement strap (velcro one, costs about a tenner - this is the important bit)
one very long usb lead (about a quid)
one mini AA battery charger (£3-4)
I run with the garmin till I see battery low. Then connect the battery pack to the long lead. Put the battery pack in my rucksack and wrap the lead around my arm. The key is the cradle fits neatly under the watch in the velcro strap. It doesnt make contact with the skin and it fits neatly. Then every few hours I ask the nice people at the aid stations to swap over the batteries.
The overall weight is minimal and it just keeps on trucking 
PS I saw the online websites with talk of rubberbands etc etc. This way you can still use the damn thing as a watch, my addiction to garmins carries on 
Edited: 01/11/2012 at 16:31