Thanks for the tips, gr5. Unfortunately, they don't appear to be the cause of my problems. There are no red or brown colours in the x-ray view, and it's not fixed by changing the line widths to 0.2mm.
I have been trying other models, including ones that I have successfully printed last week (a 20mm calibration cube), and they are also getting holes in the walls, makes it look like it's something in Cura, and not in any STL file. Could I have ruined a setting somehow?