Thanks for the replies.
Both Nooks were running outdated firmware. I updated one to the latest when I got it [1,2,1 if I recall] and the other a week or so later. Both showed the same lack of battery life, before and after firmware upgrade.
The registration issue is interesting, as I did bypass registration on both. I intended these as cheap holiday readers I can sideload my existing ePub collection onto and was never going to buy anything from B&N anyway. So that may be the answer.
Has anyone any ideas why bypassing registration would knacker battery life in this way? I could kind of understand it if wifi was on and the Nooks were periodically trying to contact the mothership. But what can be chewing up the volts with wifi off and the Nook in standby?