Put the bulgar wheat and sultanas in a bowl and pour over enough boiling water to cover by a few centimetres. Cover with a plate and leave for 10 minutes.
In a bowl, stir together the vinegar and sugar until the sugar dissolves. Add the onion, celery and olive oil, and season really well. Toss and leave for 10 minutes, stirring now and again.
Drain the bulgar wheat and sultanas really well. Tip the pepper and chickpeas into a large salad bowl and add the bulgar and all of the onion mixture. Toss everything together then add the herbs and feta, and toss again.