Adywan, just on the offchance you may not have molded plasticine with Silicone rubber before, use plenty of layers of protective clearcoat first as plasticine contains sulphur and stops silicone curing making a right ol mess! I usually sculpt from Chavant or brown terracotta wax which is very soft and meltable. no release agent needed for these. I buy mine from http://www.tiranti.co.uk/
You most likely already know all this, but that 1% chance you didn't coulda been expensive mistake.