An implant delivery system comprising: a delivery pusher; a tether having a distal portion and a proximal portion; an implant device detachably connected to the delivery pusher by the distal portion of the tether, wherein the tether is substantially non-tensioned when connecting the implant device to the delivery pusher; an electrical heater coil arranged coaxially around at least a portion of the tether, the electrical heater coil characterised by: a first winding zone having a small or tight pitch, a second winding zone having a large or open pitch, a third winding zone having a small or tight pitch and a fourth winding zone having a small or tight pitch; wherein the first winding zone is proximal of the second, third and fourth winding zones,,; wherein the third winding zone is at the distal portion of the tether, near the implant device; wherein the second winding zone extends from the first winding zone to the third winding zone; and wherein the fourth winding zone is located between the first winding zone and the third winding zone; said electrical heater coil being reversed upon itself to form a double winding region encompassing the third winding zone and a portion (Z2') of the second winding zone between the third winding zone and the fourth winding zone, said double winding region therefore having effectively two winding layers; wherein the electrical heater coil is operable to achieve a temperature in the third winding zone that is sufficient to sever the tether at a point near the distal end of the tether; and further comprising an insulating tube located coaxially around said electrical heater coil from a point distal from the third winding zone to a point proximal from the third winding zone, wherein one wire of the coil exits from the insulating tube at a point near a proximal end of the double winding region and is wound over the insulating tube to form the fourth winding zone. a delivery pusher; a tether having a distal portion and a proximal portion; an implant device detachably connected to the delivery pusher by the distal portion of the tether, wherein the tether is substantially non-tensioned when connecting the implant device to the delivery pusher; an electrical heater coil arranged coaxially around at least a portion of the tether, the electrical heater coil characterised by: a first winding zone having a small or tight pitch, a second winding zone having a large or open pitch, a third winding zone having a small or tight pitch and a fourth winding zone having a small or tight pitch; wherein the first winding zone is proximal of the second, third and fourth winding zones,,; wherein the third winding zone is at the distal portion of the tether, near the implant device; wherein the second winding zone extends from the first winding zone to the third winding zone; and wherein the fourth winding zone is located between the first winding zone and the third winding zone; said electrical heater coil being reversed upon itself to form a double winding region encompassing the third winding zone and a portion (Z2') of the second winding zone between the third winding zone and the fourth winding zone, said double winding region therefore having effectively two winding layers; a first winding zone having a small or tight pitch, a second winding zone having a large or open pitch, a third winding zone having a small or tight pitch and a fourth winding zone having a small or tight pitch; wherein the first winding zone is proximal of the second, third and fourth winding zones,,; wherein the third winding zone is at the distal portion of the tether, near the implant device; wherein the second winding zone extends from the first winding zone to the third winding zone; and wherein the fourth winding zone is located between the first winding zone and the third winding zone; said electrical heater coil being reversed upon itself to form a double winding region encompassing the third winding zone and a portion (Z2') of the second winding zone between the third winding zone and the fourth winding zone, said double winding region therefore having effectively two winding layers; wherein the electrical heater coil is operable to achieve a temperature in the third winding zone that is sufficient to sever the tether at a point near the distal end of the tether; and further comprising an insulating tube located coaxially around said electrical heater coil from a point distal from the third winding zone to a point proximal from the third winding zone, wherein one wire of the coil exits from the insulating tube at a point near a proximal end of the double winding region and is wound over the insulating tube to form the fourth winding zone.