A system comprising: an implantable medical device having an aperture with a first diameter; and a delivery system for delivering the implantable medical device, the delivery system comprising: a catheter shaft having a distal end; characterised by a tube disposed within the catheter shaft and having a main portion with a second diameter, a distal end and an expandable distal end portion extending distally beyond the distal end of the catheter shaft; and a tether disposed within the tube, the tether including an elongated body having a distal end and a tether member at the distal end of the body, the tether member having a third diameter greater than the second diameter and smaller than the first diameter, the tether being slideable relative to the tube from a released condition in which the tether member is positioned at least partially distal to the distal end of the tube, and a locked condition in which the tether member is at least partially surrounded by the expandable distal end portion of the tube, wherein in the released condition, the distal end of the tube has a diameter smaller than the first diameter, and in the locked condition, the distal end of the tube has a diameter larger than the first diameter. an implantable medical device having an aperture with a first diameter; and a delivery system for delivering the implantable medical device, the delivery system comprising: a catheter shaft having a distal end; characterised by a tube disposed within the catheter shaft and having a main portion with a second diameter, a distal end and an expandable distal end portion extending distally beyond the distal end of the catheter shaft; and a tether disposed within the tube, the tether including an elongated body having a distal end and a tether member at the distal end of the body, the tether member having a third diameter greater than the second diameter and smaller than the first diameter, the tether being slideable relative to the tube from a released condition in which the tether member is positioned at least partially distal to the distal end of the tube, and a locked condition in which the tether member is at least partially surrounded by the expandable distal end portion of the tube, wherein in the released condition, the distal end of the tube has a diameter smaller than the first diameter, and in the locked condition, the distal end of the tube has a diameter larger than the first diameter. a catheter shaft having a distal end; characterised by a tube disposed within the catheter shaft and having a main portion with a second diameter, a distal end and an expandable distal end portion extending distally beyond the distal end of the catheter shaft; and a tether disposed within the tube, the tether including an elongated body having a distal end and a tether member at the distal end of the body, the tether member having a third diameter greater than the second diameter and smaller than the first diameter, the tether being slideable relative to the tube from a released condition in which the tether member is positioned at least partially distal to the distal end of the tube, and a locked condition in which the tether member is at least partially surrounded by the expandable distal end portion of the tube, wherein in the released condition, the distal end of the tube has a diameter smaller than the first diameter, and in the locked condition, the distal end of the tube has a diameter larger than the first diameter.