A bit strange this. The UK Met Office site implies that 'NSC' is only to be used in TAFs or in the TREND of a METAR, but not in the observation of a METAR. They state:
TAFs:
Cloud:
SKC = sky clear; FEW = 1-2 oktas; SCT = 3-4 oktas; BKN = 5-7 oktas; OVC = 8 oktas; '///' = state of sky obscured (figures after '///' will give forecast vertical visibility in hundred of feet) NSC = no significant cloud (none below 5,000 feet and no CB) CB will be the only cloud type specified
Cloud heights are given in feet above airfield height.
METARS:
Cloud:
SKC=Sky clear (0 oktas), FEW='few' (1-2 oktas), SCT='Scattered' (3-4 oktas), BKN='Broken' (5-7 oktas), OVC='Overcast'. There are only two cloud types reported; TCU=towering cumulus and CB=cumulonimbus. VV///='state of sky obscured' (cloud base not discernable): Figures in lieu of '///' give vertical visibility in hundreds of feet. Up to three, but occasionally more, cloud groups may be reported.
Cloud heights are given in feet above airfield height
Trend:
BECMG=Becoming TEMPO=Temporarily NOSIG=No sig change
NSW=No sig weather AT=At FM=From TL=Until
NSC=No sig cloud
Any of the wind forecast, visibility, weather or cloud groups may be used, and CAVOK. Multiple groups may be present.
So, to my mind, it should indeed have been CAVOK!