Tesco are now offering an online price comparison against Asda. Enter your receipt details and wait for an email from Tesco to tell you if your shopping would have been cheaper from Asda. If it would have been Tesco will give you double the difference back.
You can also use the service to check your online grocery shop too.
I entered my details this afternoon an am waiting to recieve an email from Tesco to let me know if it could have been cheaper from Asda. Tesco say that you may have to wait up to 36 hours for a reply from them.