Welcome TomC!
You can try Extensions menu > Export > Plot. I believe that offers HPGL export.
Or there are 3 external extension which offer HPGL. Please visit the Home tab and scroll down to the extensions section. Open the Export category (by clicking on "Export" in the first little box) and scroll a little more down to see the HPGL section. Instructions for installing are just above the little boxes.
Or, I actually do see HPGL as a Save As option. It's right about in the middle of the menu for Save As Type field. It says "HP Graphics Language file (*.hpgl)". I guess it's possible it's there because I have installed one of those external extensions, except that I have no cutter/plotter, so it's unlikely I've installed one. But anyway, it's a big menu, so it wouldn't hurt to look again.