How can I renew the batteries in my Hilka drill battery pack (14.5 v)?

The battery pack for my Hilka drill no longer holds a charge. Hilka say its not worth buying a new pack as this would be more than a new drill. The drill is top quality so its worth spending a little time to see if the pack can be repaired.
The pack contains 12 - 1.2v batteries linked together. However these are slightly smaller than the standard rechargeable batteries available, which if used would not fit into the battery pack casing. Any Ideas gratefully received.

Regards,

Rogerinfarnham