Ordered the extended and slim Mugen batteries from ETEN. Both arrived pretty quick, but sadly both with not charge. Plug Nokia charger in and you don't get the "charging" on screen. Even leaving it for 24 hours, still apparently not enough charge in the batteries to boot the N810.

ETEN emailed to say return the batteries and they will replace them.

"Jiggled" the contacts on the 3600mAh battery and its now showing as "charging"