Wow... I'm just after finding out that the Psion Organiser II I bought runs on a 9V battery (after taking off the case - needs a little more pulling force), and it seems to work great!
This was an excellent buy for 6 quid, considering it comes with a 64K memory pack AND a games pack... this changes a lot, although I did still bid on one (with a battery which helped me realise it took a 9V) boxed because it'll cost me a fiver, plus I'm really into having boxed systems at present, for a variety of reasons; the other Psion II I get will be useful for if the other dies, or I need spare parts!
I also bid on a Psion 3a with Chess game too, mainly for the game, but having a 3a seems like a good idea too, as again it's extremely cheap!