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 . <EOS>