An orthopaedic surgical instrument assembly which includes: (a) a base cutting block comprising: a base plate including (i) a proximal surface, (ii) a distal surface positioned opposite the proximal surface, and (iii) a slot extending through the proximal surface and the distal surface, a locking tab having a body positioned between the proximal surface and the distal surface of the base plate and an ear, the body being pivotally coupled to the base plate and moveable between (i) a first position in which the ear of the locking tab is positioned in the slot and (ii) a second position in which the ear is spaced apart from the slot, and a pair of curved arms extending posteriorly from the base plate, each curved arm including (i) a posterior surface shaped to match a posterior condyle surface of a femoral prosthetic component and (ii) a cutting guide defined in the posterior surface, and (b) a first orthopaedic surgical instrument including a mounting bracket which can be mounted in the slot in the base plate, in which (i) when the body of the locking tab is in the first position, the ear of the locking tab is engaged with the mounting bracket to secure the first orthopaedic surgical instrument to the base plate, and (ii) when the body is in the second position, the ear of the locking tab is disengaged from the mounting bracket such that the first orthopaedic surgical instrument is removable from the base plate, characterised in that an aperture is formed in the base plate communicating with the slot, and the body of the locking tab is positioned in the aperture and can pivot between its first and second positions about an axis defined by a pin which extends from the locking tab through a bore in the distal surface of the base plate, the ear of the locking tab extending from the aperture when the body of the locking tab is in its first position. (a) a base cutting block comprising: a base plate including (i) a proximal surface, (ii) a distal surface positioned opposite the proximal surface, and (iii) a slot extending through the proximal surface and the distal surface, a locking tab having a body positioned between the proximal surface and the distal surface of the base plate and an ear, the body being pivotally coupled to the base plate and moveable between (i) a first position in which the ear of the locking tab is positioned in the slot and (ii) a second position in which the ear is spaced apart from the slot, and a pair of curved arms extending posteriorly from the base plate, each curved arm including (i) a posterior surface shaped to match a posterior condyle surface of a femoral prosthetic component and (ii) a cutting guide defined in the posterior surface, and a base plate including (i) a proximal surface, (ii) a distal surface positioned opposite the proximal surface, and (iii) a slot extending through the proximal surface and the distal surface, a locking tab having a body positioned between the proximal surface and the distal surface of the base plate and an ear, the body being pivotally coupled to the base plate and moveable between (i) a first position in which the ear of the locking tab is positioned in the slot and (ii) a second position in which the ear is spaced apart from the slot, and a pair of curved arms extending posteriorly from the base plate, each curved arm including (i) a posterior surface shaped to match a posterior condyle surface of a femoral prosthetic component and (ii) a cutting guide defined in the posterior surface, and (b) a first orthopaedic surgical instrument including a mounting bracket which can be mounted in the slot in the base plate, in which (i) when the body of the locking tab is in the first position, the ear of the locking tab is engaged with the mounting bracket to secure the first orthopaedic surgical instrument to the base plate, and (ii) when the body is in the second position, the ear of the locking tab is disengaged from the mounting bracket such that the first orthopaedic surgical instrument is removable from the base plate,