Problem solved! So since my last post I did a compression test, which gave results of around 165 psi across all four cylinders. I also tested the fuel pump resistance which was within spec. I noticed when I had the leads and plugs out to do the comp test that one of the spark plug tube seals looked to be sitting a bit strangely around the spark plug tube, and funnily enough the plug I removed from that cylinder had a tiny bit of oil around the bottom of it where it contacts the head once tightened up. I have replaced both the seals and the plugs not that long ago when I changed from an N15 plastic rocker cover to an N14 one to address the same issue but assumed that I'd done something wrong. As it turns out, I had installed the spark plug tube seals the wrong way around; they looked similar to a rear main seal (for example) and I assumed that they were installed with the flat edge-side out. I found out that they are meant to be installed flat-edge side first. So anyways after installing a new set of seals the right way, plus putting a new set of plugs in, she's back to driving as good as gold. The carboned-up EGR system was definitely causing issues too but that has been sorted also per my previous posts. I still can't get any fault codes to show up however!