A deflectable stylet including: an elongate deflectable member with a bend-enhancing region defined at a distal part (12.2) of the deflectable member; an actuator having a distal end fast with a distal end of the deflectable member, relative displacement between the actuator and the deflectable member causing deflection of the distal part of the deflectable member at the bend-enhancing region; a control member displaceably arranged relative to the deflectable member, the control member interacting with the bend-enhancing region of the deflectable member for controlling the extent of deflection of the distal part (12.2) of the deflectable member; and a deflection control mechanism, the deflection control mechanism controlling relative axial displacement of the deflectable member, the actuator and the control member relative to each other, and including a body member and an attachment formation at its distal end configured to engage a complementary receiving formation at a proximal end of a catheter handle of a catheter, whereby the deflection control mechanism is configured to be mounted to said proximal end of the catheter handle; wherein the deflectable stylet is adapted to be received within a lumen defined by an electrode sheath of the catheter, the electrode sheath being attached to a distal end of the catheter handle; characterised in that the deflection control mechanism includes a mounting formation configured to engage a complementary receiving formation of the catheter handle, the mounting formation of the deflection control mechanism is displaceably arranged relative to the body member, the mounting formation is configured to act on a deflection control element of the catheter handle and further to mount one of the deflectable member and the actuator, and the body member of the deflection control mechanism defines an anchoring formation configured to anchor the other of the actuator and the deflectable member. an elongate deflectable member with a bend-enhancing region defined at a distal part (12.2) of the deflectable member; an actuator having a distal end fast with a distal end of the deflectable member, relative displacement between the actuator and the deflectable member causing deflection of the distal part of the deflectable member at the bend-enhancing region; a control member displaceably arranged relative to the deflectable member, the control member interacting with the bend-enhancing region of the deflectable member for controlling the extent of deflection of the distal part (12.2) of the deflectable member; and a deflection control mechanism, the deflection control mechanism controlling relative axial displacement of the deflectable member, the actuator and the control member relative to each other, and including a body member and an attachment formation at its distal end configured to engage a complementary receiving formation at a proximal end of a catheter handle of a catheter, whereby the deflection control mechanism is configured to be mounted to said proximal end of the catheter handle; wherein the deflectable stylet is adapted to be received within a lumen defined by an electrode sheath of the catheter, the electrode sheath being attached to a distal end of the catheter handle; characterised in that the deflection control mechanism includes a mounting formation configured to engage a complementary receiving formation of the catheter handle, the mounting formation of the deflection control mechanism is displaceably arranged relative to the body member, the mounting formation is configured to act on a deflection control element of the catheter handle and further to mount one of the deflectable member and the actuator, and the body member of the deflection control mechanism defines an anchoring formation configured to anchor the other of the actuator and the deflectable member.