Seat Ibiza engine cutting out when decelerating

Recently acquired this vehicle, engine intermittently cuts out when decelerating, mostly when pulling up at roundabouts & junctions, usually restarts but not always, there has occasionally been a strong exhaust smell when it does cut out, have been told it could be engine crank speed sensor, but when I rang parts factor there are 4 of them, 2 at the front and 2 at the back, how do you test for the fault and decide which one is faulty ?.
It has also been suggested that it could be the coolant temperature sensor.
Help please.