Itinerans? Are you sure?
Too many people from the UK say they have killed dozens of these.. Stupid question from my side, but sure you don't mean musa basjoo?
I have heard reports from England stating that Itinerans is just borderline for the UK climate, but I guess it depends on if your'e in the Isles of Scilly or in Yorkshire.
As for sikkimensis, the "Red Tiger"-type seems not that vigourous compared to my regular ones. Same goes for "Darjeeling Giant". But Helens' hybrid beats them all in terms of growth.