Hi, funnily enough when I learned how to see the layers in the slicer I could see those lines are actually present as part of the fill, so that part of the issue is solved. I’m still not sure what is causing under-extrusion but have a possible culprit; the hot end cooling fan wasn’t working. Once I’d calibrated everything following the earlier responses, I started getting severe blockages in the hot end so a test cube would be okay for the first 2mm or so but then start to fail such that, within another couple of layers, nothing was getting laid down. The retailer has sent me new hot end fans and once I’ve fitted these and some Capricorn tubing (The original is now a bit short, from cutting off the irretrievably blocked section) I’ll see what happens..