So here's the method I ended up using: Zoom into the image as far as possible and pan around to load all high resolution chunks (not entirely sure if this is one hundred percent necessary). Enlarge the canvas size by using the first three lines of the script:

var container = document.querySelector("div.articlePage.container");
container.style.width="6000px";
container.style.maxWidth="6000px";

Right-clicked on the canvas element in the inspector and chose Copy> 'Image Data-URL' (as shown in the 'Canvas Access' section) to get the Base64 version. Pasted that into the address bar (in Firefox - presume it works similarly in other browsers) and pressed enter. Right-clicked the decoded Base64 image and saved the image as normal. Not quite as automated as the original method, but I got the high-res images.