Battery Replacement Kit for Sony PRS-350 - £15.95
Battery Replacement Kit for Sony PRS-650 - £21.95
Thanks to the OP, I'll replace my newpower99.com bookmark with this one. Although going by the few reviews already on Amazon, I'm not too hopeful of how much of a difference they'll make if and when needed. One of the four star reviews for the 650 battery states "It's only that my reader still is empty in a day, so probably it was not the battery but the reader."