I would just let it go, it's not worth fighting about. Waitrose will deny it and say you planted it there. Move on you can't win this one.
Enjoyed this banter and a nice 'antidote 'having just 'endured' the vitriolic posts in threads about the Royal Wedding.
The only way to handle this situation is to return to the store and go bat **** crazy by throwing domestos all over their green vegetables. Unfortunately you run the risk of getting nicked for bleach of the peas.