Seat Cordoba Diesel Lean Running Issue

Lean running on a diesel? That points towards a fuel delivery problem. Have you checked the mass air flow sensor readings? Is the catalytic converter functioning properly? Perhaps the throttle body needs a clean?

Regarding the mass air flow sensor, what would be considered 'normal' readings? And could a faulty oxygen sensor also contribute to this, even on a diesel?

Normal mass air flow sensor readings depend on the engine load, but wildly fluctuating or consistently low values are red flags. Yes, an oxygen sensor issue could contribute, though less common on diesels. Given the symptoms, and your injector suspicion, a visit to a workshop for proper diagnostics seems wise.

As suspected, the fuel injectors were indeed the culprit. Replaced them all, and the lean running issue is resolved. Total damage was 515€.