Seat Arosa Starting Issue

This sounds like an ECU issue. Given the symptoms, a faulty connection to the ECU is a strong possibility. You mentioned the battery connections, that's a good place to start since the ECU needs a stable power supply. Also, check the earth straps around the engine and gearbox. Are you experiencing any engine stalling or a check engine light appearing on the dashboard? A diagnostic scan could reveal a fault code, pointing to the cause of the problem.

Thanks for the quick reply! When you say 'earth straps' are you referring to the ground cables connecting the engine to the chassis? And yes, the check engine light was on before it stopped starting altogether. Where would I plug in a scanner to check for error codes on this Arosa? Is it something obvious?

Yes, by 'earth straps,' I mean the ground cables. On your Arosa, the OBD II port (where you plug in the scanner) is usually located near the steering wheel, often below the dashboard. If the check engine light was on, there's definitely a stored code. If cleaning the battery terminals doesn’t fix it, and you are unfamiliar with using the scanner, I recommend taking your car to a workshop for professional diagnosis. Replacing the ECU can be expensive, and you need to be absolutely sure that it is in fact an ECU failure before you go down that route.

Just wanted to let you know, you were right! It was the battery cable connections. Cleaned them, and it started immediately. I took it to the shop for a proper check, and they confirmed it. Cost 155€ for the diagnostic and cleanup. Thanks so much for your help!