ok i'll try again, this time i'll cite page numbers cos my last explanation was a little confusing even to me
slow and perposeful gives you the highest of 2d6 in the movement phase in the same fashon as a difficult terrain test (pages 76 and 17 of the bbb).
page 33 of the chaos dex, under the spawn entry, it says "unit type:beasts", which gives them a fleet move (page 57 bbb). fleet gives a further d6 in the shooting phase. and as beasts they get a 12" charge (also pg 57 bbb)
so, (highest of 2d6)+d6+12=14-24"