OK - I'm just about familiar with joins, link IDs etc, but was just keeping it simple here as per the instructions to get the dropdowns working.
So in the Products table I have the fields Main_Category and 1st_Sub which correspond to category in the Categories table and SubCategory in the subcategories table.
With the join, do I need to link the Products table to the categories and/or subcategories tables in the Parent and/or Child recordsets?