The 2012 NCT Manual states in part
Page 60 "2. Examination is confined to tyres fitted to the road wheels. If, however, any defect on a tyre carried on a spare wheel is seen, the driver should be advised." and
Page 61 "3. Check that each tyre fitted on the vehicle has an E-mark or e-mark"
Spare is advisory only, and only if defects or deviations from spec are seen. The check on the spare is just to be sure you have one, if you don't have run-flats / tyre-weld.