The Rio US knowledgebase
"Batteries for players having hard drives, such as the Nitrus, Carbon, or Karma are not serviceable. We do not sell or service batteries.
Please charge the player for a minimum of 3 hours using the AC charger, when the player is not in use. If you are using a Carbon series player, please do not use the USB to charge the player. Please refer to the Knowledge Base for more trouble-shooting steps.
If the battery still fails to function properly, then it may be necessary to have the player repaired or replaced. Please refer to the link below for warranty information."
And then gives a link in the States. The UK warranty offered was 1 year from purchase, so It looks like you may be out of luck. You could try hunting on the Riovolution
enthusiasts' site, but even there the news looks pretty grim.
The Rio Europe HQ contact information is given as
Kingsbridge House, Padbury Oaks, 575-583 Bath Road, Longford, Middlesex UB7 OEH
Telephone +44 (0) 1753 680022