LineUpAndWait and Deano, thank You for filling the gap here. Of course, the figures I stated are the absolute minimum to which the aircraft is certified - lower than that is not flyable on the DH8-400 in my company. But as has been stated, the use of those minima is dependent on the availability of Cat 3 (or Cat 2 in that case) suitable ground equipment, currency and certification of the flight crew and serviceability of the relevant aircraft systems.
So - to determine the lowest USABLE minimum of the DH8-400 (and any other aircraft I am aware of) for a specific runway, look at
1. the lowest minimum the aircraft is certified for with regards to systems installed and serviceable;
2. the lowest minimum both of the operating flight crew members are trained and certified for (of course, if only one has Cat III training while the other one has none, no Cat III can be flown in anger);
3. the lowest minimum the airfield/runway to be used is equipped and certified for - usually to be found out on the approach plates, but keeping in mind possible temporary downgrades of the equipment most often disseminated via NOTAMs. Also an ILS transmitter or the runway lighting might fail sometimes.
And from those 3 sets of figures, You will take the highest as Your requirements for the approach and then fly the associated approach procedure.