Found it. Further to above from the supplied SDC2300 charger (enecharger NC1500U) specs/instructions:
"Built in safety timer - For the 4 main slots (not 9V) if charging is still taking place after 3 hours the safety timer will cut out the fast charge and the charging current will drop to trickle charge rate. For high capacity batteries (higher than 4.4Ah) the charger will need to be reset (turned off then back on again after 3 hours) for fast charging to resume"
This is likely why some people are having issues with short battery life etc. from this charger.
To get a full recharge you would need to reset the charger as per above as the batteries supplied are 5000mAh (5Ah).
I knew I'd seen it somewhere but as I used a different 8 bank smart charger I never had this issue myself. I'll sleep better tonight now