Paid £10 yellow label on box last week

How do you know that?

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

what barcode

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

what is the app called

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....