E.diphyllum x E.grandiflorum.
I am not quite sure that this is the proper name to apply, but it is a distinct pale lilac form, and a distinct name, so the two things might as well
stay together for the time being. The 'typical' form of the hybrid has already been named E. x youngianum 'Youngianum', historically important as the original clone of
the hybrid that was named. It has pure white flowers, so this isn't it!