my baby also has very serious rash on the neck, i tried the following and the rash totally disappeared within 2 days.
My baby's doctor has been giving him Mustela's hypoallergic bath oil (this line is for sensitive skin, the packaging is light blue, not dark blue) for his ezcema. Twice a day I wipe his neck with the bath oil using a cotton ball, leave it on for ~5 mins, then use another cotton ball to rinse it with boiled water, pat dry and apply Silic 15 cream. This works wonders for my boy whenever he has skin problem. I hope this helps.