Seat Inca Fuel Consumption Issue

Sounds like VANOS failure to me. Had similar engine misfire symptoms and check engine light on my BMW years ago. Turned out to be a faulty VANOS solenoid. Have you checked yours?

Where exactly is the VANOS solenoid located, and is there any way to test it before I potentially waste money on a replacement?

The VANOS solenoid controls the oil flow to the VANOS unit, affecting inlet camshaft timing. Location varies, look around the valve cover, usually near the camshaft gears. Testing involves checking resistance with a multimeter or swapping it with a known good one if you have access. Given the symptoms, a workshop visit might be the best bet to properly diagnose it and avoid further issues.

Took it to the shop. You were spot on, VANOS solenoid was the culprit. New one fitted, check engine light's gone, and the fuel consumption seems back to normal. Cost me around 250€. Thanks for the help!