An orthopaedic surgical instrument for use in preparing a femur in a knee replacement surgical procedure, comprising: an A/P femoral cutting block having (i) an anterior cutting surface, (ii) a posterior cutting surface, (iii) a chamfer access window positioned between the anterior cutting surface and the posterior cutting surface, and (iv) an interference-fit connector (56, 58), and a chamfer femoral cutting block having (i) a wedge-shaped chamfer cutting surface positioned in the chamfer access cutting window, (ii) an interference-fit connector connected to the interference-connector of the A/P femoral cutting block so as to removably secure the chamfer femoral cutting block to the A/P femoral cutting block, (iii) a bone-engaging surface opposite the wedge-shaped chamfer cutting surface, and (iv) a number of spikes extending outwardly away from the bone-engaging surface, in which the chamfer femoral cutting block can be displaced relative to the A/P femoral cutting block from (i) an assembled position in which the wedge-shaped chamfer cutting surface is nested within the chamfer access window in the A/P femoral cutting block, to (ii) a separated position by application of force to break the interference fit connection between the cutting blocks, characterised in that the bone-contacting surface of the A/P femoral cutting block is able to contact the distal surface of the femur in a selected position when the chamfer femoral cutting block is in its assembled position in the A/P cutting block, allowing the position of the A/P cutting block to be changed on the distal surface of the femur, and the spikes on the chamfer cutting block can be driven into the bone tissue on the distal face of the femur while the bone-contact surface of the A/P femoral cutting block is in contact with the distal surface of the femur, by the said application of force to break the interference fit connection between the blocks. an A/P femoral cutting block having (i) an anterior cutting surface, (ii) a posterior cutting surface, (iii) a chamfer access window positioned between the anterior cutting surface and the posterior cutting surface, and (iv) an interference-fit connector (56, 58), and a chamfer femoral cutting block having (i) a wedge-shaped chamfer cutting surface positioned in the chamfer access cutting window, (ii) an interference-fit connector connected to the interference-connector of the A/P femoral cutting block so as to removably secure the chamfer femoral cutting block to the A/P femoral cutting block, (iii) a bone-engaging surface opposite the wedge-shaped chamfer cutting surface, and (iv) a number of spikes extending outwardly away from the bone-engaging surface, in which the chamfer femoral cutting block can be displaced relative to the A/P femoral cutting block from (i) an assembled position in which the wedge-shaped chamfer cutting surface is nested within the chamfer access window in the A/P femoral cutting block, to (ii) a separated position by application of force to break the interference fit connection between the cutting blocks, characterised in that the bone-contacting surface of the A/P femoral cutting block is able to contact the distal surface of the femur in a selected position when the chamfer femoral cutting block is in its assembled position in the A/P cutting block, allowing the position of the A/P cutting block to be changed on the distal surface of the femur, and the spikes on the chamfer cutting block can be driven into the bone tissue on the distal face of the femur while the bone-contact surface of the A/P femoral cutting block is in contact with the distal surface of the femur, by the said application of force to break the interference fit connection between the blocks.