Thanks for the reply.
Forum would not allow me to post link so I've attached two screenshots.
newbielink:https://test.stationroadbaseboards.co.uk/cart_template_20.htm [nonactive]
Page is coded to enable the menu to stay visible at the top of the screen at all times.
In the first one if “add to cart” is clicked before scrolling the page the box sits above everything else, as intended. In the second picture, scrolling the page first, until the menu sits at the page top & then clicking “add to cart” results in the box sitting behind the menu.
I've tested on Firefox, Chrome, Edge & Opera, all with the same results.