She isn't from working lines, I got her from a show breeder.
I have been working hard on anticipating what she is going to do based on her body language as Wildmoor suggested and she is getting much better, in fact, last night she didnt react to any cars and we managed a short and very slow walk to heel.
Stuart was dead right in saying that I hadn't been as consistent as I should have been. I have now upped my game!
BB - great suggestion

I have been taking her to the supermarket since she was 9 weeks old, my back was killing me after weeks of carrying her around! I didnt concentrate on cars though - I didnt anticipate that one. Wonder what else I should be ready for!