You can only make a citizen's arrest if an arrestable offence has actually been committed. A police officer may make an arrest merely on suspicion.
Suppose the "thief" had an informal arrangement that he could pop in and take crisps and pay for them later, he would not be stealing. If a citizen arrested this person he could be sued for false arrest. A police officer could not.
Bearing this in mind, I probably would take no action.

I actually had such an arrangement with an antique dealer. He left some statues outside his shop for me to collect. I did so under the gaze of a police officer, who did not even come over to talk to me. But he did note the registration number of my car.