All I recall is that we used to use H mode a lot (NO beacon - whoever came up with that nonsense) especially after a winter nightstop in Luxembourg. Bl00dy aeroplane would be freezing cold and that No2 engine heated it beautifully for approx 45-60 mins before ETD. There was no time restriction on H mode - just as someone has pointed out the nacelle overheat might get a little fruity
Great aeroplane the 42 and 72..