so it would be OK in sealed plastic bag in a shed on the top shelf
with no other chemicals
Yes,as long as it's airtight your fine(with most chemicals) but,you can do alot better than a plastic bag!!Also make sure you keep oxidizers,organic fuels,metallic fuels and additives apart from each other!.
I should imagine (or hope) that as your starting out you only have 3 chemicals (potassium nitrate,charcoal,sulphur)but,as you gain knowledge and experience,so will your chemical collection and pretty early on,you'll need adequate storage.
My advice is to buy some airtight storage bottles (www.inoxia.co.uk) and either buy or build some cupboards so you have separate storage for your chemicals ( oxidizers,fuels).Also,read as much as you can about the chemicals you intend to be working with before you do,that way you know what your dealing with and what SAFETY PRECAUTIONS! you need to take.
P.S Use the search function,it's all here
dean