cormac , there are other members who managed to replace their batteries with ipod nano replacement batteries. However, every player differs in hardware, so some might fit, some might not.
What i can suggest is that if you want to try replacing the battery first. Open the player and measure the max allowable space for the battery. Then go onto ebay to look for the nano replacement battery.
You can read this thread for further details on the replacement:
http://www.mympxplayer.org/new-batte...ne-vt2313.html