The problem isn't with the materials at all, as I've used the same method to get some chest and shoulder armour off my sculpts to great effect.
The problem comes that with a rigid one piece mould wrapped around a helmet, I can't remove it without breaking the rigidness of the mother mould, which is near impossible to reattach to form up the shape it moulded, hence the latex flops about and the shape is lost. I've though about attempting a two piece mould, or to have another method of doing a one piece mould - some trick of the trade someone's used to great effect.
Sorry to confuse anyone over that 
