If the CTXD is still discharging cell #6, the unit may need to be recalibrated. Have you tried to discharge your packs on another CTXD or CTXD2? As or8ital said, you definitely don't want to charge the pack and risk the cell exploding.
Have you flipped your fans around? If not, give that a try and see if you still get the low reading on cell #6.
If the CTXD does need repaired, you will need to contact Muchmore UK for service. MM did have upgrade kits available at the time of the CTXD2 release, but you would need to check with MM UK to see if they still have any available. The upgrade does not include the extra posts, it's a internal upgrade.
Retired from RC....
Quantum - St. Croix - Bass Pro Shops - XPS - Rapala