As has been said, the 1.4 engine is an old model and, although generally solid and reliable, it has the old-world economy of that generation. The economy you indicate seems about right, though if you want a totally accurate fuel economy figure you'll have to do a "brim to brim" manual calculation. The trip computer is reasonably accurate, but I wouldn't rely oin the "distance to empty" readout - it is normally pessimistic - probably on purpose so you don't run out of fuel and complain to Seat!
As regards the Engine Management light, does this happen frequently? Try to video the fault happening using your phone so you have documented proof. I'd be tempted to drive straight to the dealer if the fault happens within working hours and you have spare time to do it. Then let the Service Manager witness the fault in progress... They can't then simply say that because there's no fault code showing, they can't treat the issue seriously! I had a similar fight with my local Seat dealer when the trip coimputer totally died one day, then came back the next! No fault code so no action, and they attempted to bill me for the work!