This is a fairly common discussion topic on forums at this time of year. It’s often discussed on VW forums - presumably they have the same basic heating, ventilation and A/C systems as Seat - although I’ve also seen it discussed on BMW, Mini, Toyota forums as well, so it’s not just something that affects VAG vehicles.
I have the climate control in my VW set to Auto and it’s on 24/7, and I don’t experience the condensation issues that others seem to. As
@ZK_FR150 has said, ensure you keep the inside of the screen really clean. It’s also worth checking for blocked pollen filters, and for anyone with a sunroof it’s worth checking for blocked sunroof drain pipes that might cause water to enter the car’s cabin. Also, remove any outdoor clothing
store or umbrellas from the car that might be wet / damp and contributing to any condensation build up issues.
Edit; also worth checking carpets and mats for dampness. If they are damp, then check door seals to ensure they’re not damaged and letting water into the cabin. Some owners of the mk7 / mk7.5 Golf have had issues with water getting into the cabin through defective seals around the door mounted speakers. Don’t know if defective speaker seals are an issue with the Leon, but for anyone with damp carpets that might be causing condensation issues, it’s worth checking.