Gearbox end is held by a knurled knob screwed to the gearbox drive.
Its been some time since I removed Sporty dashboard. I recollect [not certain because there might be a single screw holding the outer cable to the speedo] simply push the outer flexible cable into the speedo whilst locating the driven cable into the speedo central gear/hole. The centre cable has a square end so it might be awkward locating the centre cable whilst pushing the outer cable into the speedo. The problem is getting your hands up behind the dashboard. Possibly made easier if the cable is detached from the gearbox to allow the inner cable to be fed into the speedo after the outer cable is attached to the speedo. When the outer is attached to the speedo, twist the inner cable at the gearbox end and push into the outer cable, until you feel the square inner cable locate. Refit the knurled knob onto the gearbox drive.