Just a note that I have contacted Rob, as to my current knowledge N. 'Helen' has not been released except for a few plants available at Chelsea. BE does have a mix of clones from tc of the N. spathulata x spectabilis cross available. These however are not the N. 'helen' cultivar.
Oh almost forgot.. it would be N. 'Helen' not N. x 'Helen'. N. 'Helen' pertains to a single selected cultivar from the cross and is not a grex/hybrid name for any N. spathulata x spectabilis plant.