Pumping the primer resulted in no noticeable effect.
Jacked the car up to take a look at the loom. It's in hard(ish) corrugated tube and looks just fine. Any closer inspection is definetely not a job to do outside, swimming backstroke under the car, in these temperatures. It's been awfully warm lately and only about -9 C today, though.
Found this under the rubber cap.
injectionpumps.co.uk says it's an advance solenoid. Does it act as a stop solenoid, too?
I don't know what to call the other part in the last photo, but the o-ring under it was hardened. The other o-ring was in better condition, but they both seemed quite thin to me. Anyway, I picked two similar from a local parts store. They said, they're not really meant to be in touch with diesel, so I'll have to replace those with something decent.
Made a really quick test run and at least it's not any worse. We'll see.
Solenoid has some weird marks on it, as seen in the second photo above the o-ring. What could that be? Corrosion?
Oh, and I somehow forgot to mention, that there's been occasionally a significant amount of white/blue smoke from the exhaust. It's not burning too much oil. I guess air in the fuel system and/or something electrical could cause that?