Chicken and egg?

It is said that having a Waitrose in the vicinity can increase the value of a property by some £3,000. But is this supported by the evidence?  Can anybody show that the value of a particular property pre-Waitrose was £x, and the value of the same property post-Waitrose was £x + 3000-odd?  I think not.  What seems to be being said is that the value of properties in he vicinity of a Waitrose are in general some £3000 higher than the average for the area.  But couldn't it just be that Waitrose choose to site their stores in the more affluent localities and that house prices simply mirror this affluence?