As I mentioned, I have tried it all. Dentinox made it worse and both the olive and coconut oil started to give off a rather hideous fermented cheesy odour. It was so bad that it even put the Grandmother off cuddling her. Pretty indicative, wouldn't you say?
I was doing my usual over researching thing and came across the Mustela brand. In particular lots of positive reviews regarding the foam shampoo and serum. It took only a week of using both products to rid Marni of her cradle cap.
I couldn't specifically say which of the two really got it under control. I'm certain that both combined are the most effective form of treatment. Now that the episode has cleared I just use the shampoo at night and the cradle cap has yet to return. Hurrah! Grandmother hugs can commence once again.