S!
Crumpp, still some 17 years left of service, going for the full service time
Anyways, when looking at that Spitfire Mk.II manual June 1940, paragraph 55 (stating it should be carefully noted) clearly says +12lbs up to 1000ft for take-off or maximum 3min. Rest seems to be +9lbs (all-out for 5min) with no alt restrictions. And continuous/max cruise is +7lbs. So that pretty much says it all IMHO.
Cleared for +12lbs but not for 5min or at any altitude. As that note says 5min is for +9lbs. Out of curiosity how different is the Merlin III used in Spitfire Mk.I being able to use+12lbs for 5min than Merlin XII on Spitfire Mk.II only cleared for take-off up to 1000ft / 3min? Devil is in the details it seems..