This post relates to last week's puzzle and thread, but I couldn't submit it then because it includes the actual solution.
In my engineering days I often used grubscrews, but I don't recall ever having to write it down. However, had i needed to I am fairly sure that I would have written it as a single word without thinking, and certainly without reference to a dictionary. Having now done so, and finding that they, and even my ancient Machinery's Screw Thread Book, give it as two words, I still think that one word is better. A "grubscrew" is a special type of screw, small, headless, and with a special tip, so-called I suspect because it resembles a grub. A "grub screw" seems to me to imply a screw that has some function relating to a grub, which is difficult to imagine. So I am with Imogen on this.