Tesco rarely works like this when it comes to Lego pricing. They are often store specific, or specific to a certain number of stores, even when on the white shelf labels. This, however, could still be national....

what is the app called

Several stores have it for £15.99 that's how they know that

what barcode

Stock check app has it that price for all the extras I've checked. Normally how Tesco works.

How do you know that?

Paid £10 yellow label on box last week