I'd take it back to the people who serviced it and get them to sort it?
When I lost the code for my Astra radio, the local car audio place got the code for about £20. But I don't know what the interaction is between a Seat stereo and its ECU.
I know that Seat can trace the code back for you, but I don't expect it to be cheap, or quick. See if you can find somewhere that specialises in car audio to see what they suggest.