How is it possible to add either a background image or watermark to the first
page only of a word template?

The image is A4 sized and covers the entire page, first page text will be
layered on top of the image.

I have created a template for a company, they sometimes only need the one
sheet of A4, which is fine, but sometimes they need 3 or 4 pages, and only
require the background image/watermark on the first page of the set.

Surely this simple - every day task is easy enough? I have been racking my
brains for over an hour now.