RobbW, thank you for the update.
So far, Mitsubishi has been very good about replacing battery packs which exhibit the problem you're experiencing.
To me it's unclear whether that single cell problem is a defective cell (most probably) or the BMS as it applies to that cell.
For Mitsu, it is an expensive process as they ship the replacement pack to the dealership with all the special equipment to remove/replace the pack.
We have had at least one member successfully replace a defective cell in his pack - Martin (me68), developer of CaniOn. detailed in this thread:http://myimiev.com/forum/viewtopic.php?f=23&t=2720&
Replacing an individual cell with one of similar capacity to the rest of the pack is another variable - to me, off the top of my head I don't see why a 'better' cell would necessarily be a bad thing, recognizing it is a series string of 88 cells.
This is quite a dilemma for Mitsubishi, as, although the cell replacement itself is fairly straightforward, the technician education and OSHA constraints when working with high voltage are formidable enough that only pack replacement at dealerships is presently viable.
RobbW, based on my and others' past experience, Mitsubishi will take care of you. In the mentime, have fun driving your poison-gas-spewing ICE loaner.