Andy,
I'm somewhat confused

In that thread you posted, Pagercam has explained pretty clearly what an HQI ballast/bulb is... take another read of that thread... after reading that thread myself, it explains a lot...
It appears that you originally purchased a sodium bulb and ballast (from a hydroponics shop ?)... the soduim bulb would produce a very yellowish light, which you would want to change (understandably). I believe that NAV gear is also refered to as SON gear, which is what your original coil/core ballast is... therefore, the bulb you purchased (Osram HQI) will run just fine on that ballast... I would also hazzard a pretty good guess that the LL eballast would run that bulb... but I still can't be 100% certain.... then again, no need for the LL eballast as you already have a ballast designed for the blub...
cheers,
gs