2003 Volkswagen Polo not cranking

During our snow season, was a bitch to start, but after much jumping, would fire up.
Now good weathers here, engine wont fire up. On its last working journey, it was chugging/holding back as if firing on 4 pots, but only when you touch the throttle...tick over was fine.Immobiliser light flashes, and as suggested by RAC, crank sensors been changed, and new key transponder. Does the system have to have faults reset with a scanner before they'll work again? The symptoms on non cranking till now have been a slurring, indicating the crank sensor at fault, which throws out the timing sequence. Still does slurring with new one fitted, hence my question of has it all to be reset before working properly. thanks