I have the Nectar App but cannot see anywhere how to access this.Does it come up automatically if you visit Sainsburys and don't use any App based bonuses?
Yes, if you visit Sainsbury's and don't use an App based bonuses, then when you open up the app after the store transaction and it pops up automatically with a message asking if you would like to play the bonus game now. (I usually rub the screen while pushing the trolley back to the car, it is available almost instantly after the transaction).
One note of caution - If you have multiple nectar cards on your account, if you use the primary card for the app then you must use the same card for the transaction in store. Using a supplementary nectar card on the same account for the transaction will not trigger the game (nor the in app bonuses for that matter)