Here you go mate, the letter of the law itself as regards kneeling, according to the BFTA Core Rules anyway;
"b. Kneeling: There shall be only 3 points of contact with the ground (2 feet and 1 knee). The rear foot shall be upright and straight in line with the knee. A legal seat may be used to support the rear foot and/or ankle, or to keep the knee clean provided that the foot has contact with the ground. The leading hand will support the gun and, forward of the wrist should itself be unsupported."
Hope that clears it up for you mucker.