Thank you for the hint. You suggest we should create a new table called "productcategories" which relates to the "categories" table, similar to the method which the "productoptions" table relates to the "options" table.
We would then have to change from a simple drop-down category selection in the productinsert and productupdate pages to a new page where the user can select multiple categories, like they would for options, which would be assigned to the product.
I am not sure how to link the tables together to allow a single product to have multiple category assignments.
Any further assistance is greatly appreciated.