I've never shopped online with Sainsbury but have with tesco. I can't understand why they haven't already refunded the money .
I would send them another email, stating how unhappy you are with their service. Tell them that you want a reply to your email within 24 hours, and that you want a refund.
Also, if possible, visit your local store to speak to the manager. It will be harder for them to ignore you in person.