Apparently if they don't have the item you have ordered in stock they will give you a substitution of equal or higher value. I have noticed that if you order something from their value range that isn't in stock they claim to not have a substitution (presumably because the alternative would be quite a lot more expensive for them).
I would much rather be offered the chance to pay for a higher priced item than have to go out to the shop to get something essential like bread which kind of defeats the purpose of having your shopping delivered to your door.
And no Tesco - I don't believe you that you had no pasta in the entire shop to substitute for the value pasta I ordered today or no baby wipes of any description the other week.
I'm seriously considering ordering from a different supermarket as it's an absolute waste of time getting home delivery and then having to go out to get the bits they 'haven't got'. Are any of the other supermarkets any better?