Earlier (and now redundant) smartphones might be a good choice. Older Nokia phones have a good reputation for battery life, and a Nokia 5800 smartphone can be picked up secondhand on fleabay for £30-40 (get an unlocked one, or if technically-minded hack the phone, then you can use any SIM card).
As Mur mentioned above, airplane (flight) mode saves battery life - you can get 8-10 days life from a 5800 battery. With judicious use of GPS, ebook reader etc, a field battery life of 4-5 days is the norm. Third party batteries are cheap, but also variable quality - see here and here for info and good choices. With reasonable batteries at £2.75, you can take a few...
A simple app for displaying OS grid coordinates is "GPS coordinate converter" in Nokia's Ovi store here, see also here. This can be used on demand. TrekBuddy is a good & free "moving map" navigation app if you wish to upgrade at any time.
A good ebook reader is ZX Reader or Alternate Reader. Put the text to white and the background to black to save battery life, and read in the dark without a headlight! The screen is small, but it is do-able.
Edited: 25/06/2012 at 13:41