The latest evidence states that these helmets do not change the shape of babies heads but do increase the incidence of complications (all parents using the helmets in this study reported adverse effects ranging from a bit of a smelly head to significant heat rashes)
www.boston.com/health/2014/05/01/study-corrective-baby-helmets-don-work/xDQ7n2q66rMJhAVEPmVbWP/story.html gives a nicely written account of this research.
Mumsnet hq, why are you still promoting companies who the latest evidence states only makes babies harder to cuddle and a little stinky, for at least 2grand a silly hat?