Berlingo diesel generator issue

Sounds like you have a voltage drop somewhere. The reduced power, warning light, poor starting, and flickering lights all point to that. When the generator, or alternator as some call it, isn't producing enough power, the car starts pulling power from the battery. That causes all sorts of problems. Have you checked the voltage across the battery terminals when the engine is running? It should be around 14 volts. Also, have you tried jump starting it, and have there been any issues?

Voltage across the battery terminals? I haven't done that, to be honest, but it sounds like a good start. Where would I even begin to check the voltage across the battery terminals, as I would not want to damage anything? And with jump starting, what should I be on the lookout for?

Checking the voltage is pretty straightforward. You'll need a multimeter, set it to DC volts. With the engine running, put the probes on the positive and negative terminals of the battery. If it's significantly below 14 volts, the alternator is likely the culprit. With jump starting, if it starts immediately and runs fine while jumped, that further points to the generator not charging the battery. However, given all the symptoms, and the possibility of a defective generator, your best bet is probably getting it to a workshop to confirm and possibly proceed with an alternator replacement.

Thanks for confirming what I was suspecting. I took it to the workshop, and you were right; the generator was toast! A new one plus labour cost me 960€. Ouch. But at least it's running properly again now. I’ll definitely check that voltage drop myself next time!