all lofts that i insulate have 100mm snuggly between the joists then between 150mm to 200mm cross layed over the top covering all loft space (ceiling). also apply same thickness 250mm to 300mm to loft hatch if possible securing it by enclosing within a black bag for example then apply draft...
the best way or easy way is to pull up floorboards as this is very similiar to laying insulation in loft and ensuring to fill whole space which prevents cold spots but then also stops the heat from rising from one level to the other hope this helps rich.