Well, the SLAB that was reading 5V has definitely had it! Initially, it looked good, and I was able to transfer it to my Accucel charger. But then, the voltage started to drop, even on charge. It was always unlikely that this one would recover.
The one reading 10V, I was much more conservative with the water (about 2ml per cell), and have left this on the slower charger. Its still only taking a very low current, just a handful of mA, but this is steadily increasing.
So, who knows?