Palito

Just looking for an opinion. Mine MK 1 Fabia does this too when warm. Revs stay where they were when driving and do not go down until I push acceleration pedal to the floor or slow down while in gear. They guy that services my car said I need a new "throttle". That is a direct translation from my native tongue. That would cost around £400 for new part. I am not sure my Fabia is worth that much. What would you guys suggest to check first to make sure this can be done right and with fair pricing.