Another way of doing this might be to audit payments of consumers —card processors, banks or some government layer over the top— to select 50 customers a month

Another way of doing this might be to audit payments of consumers —card processors, banks or some government layer over the top— to select 50 customers a month from a business and offer them each £5 cashback for a review. It's out-of-loop, some time after the incident, ideally anonymous (so no other kickbacks). They might be able to do the same for products.