Hello Mark,
Did 'Winter Sun' survive last winter for you? I'm interested, as I've just bought a plant of it from the Jacques Amand stand at the Malvern RHS Show. I've been asking around, and it has been suggested that it is a hybrid between N. undulata & N. bowdenii (the more commonly grown 'Pink Triumph' has the same parentage, as does the rather rare 'Lord Grenfell').
Cheers,
Malcolm