I refitted the old cable but as the old clutch had worn down and the cable had adjusted shorter to compensate, the cable would have been too tight, bringing the release bearing into play without touching the clutch pedal; this would result in clutch slip.
To close out this thread, this is how I managed to reset and extend the cable which, when re-fitted, self adjusted to the new clutch correctly:
1.Leave the cable attached to the clutch pedal end
2.Get a large screwdriver or pry bar and wrap the end of the inner cable (Gearbox end) round it and get a firm grip
3.Tug the inner cable sharply in the opposite direction to the outer sleeve keeping the cable as straight as possible. You will hear a 'zip' sound similair to doing up a cable tie.
4.Keep hold of the inner cable as if you let go before attaching it to the selector lever, it will self adjust inwards again
5.Once attached to selector lever, the cable will self adjust
6.Check operation of clutch pedal and biting point