I doubt very much if BA could in current times employ mystery shoppers
Why?
Because BASSA would object to the `spy in the sky`
BA rely on GPM surveys from customers and things like customer forums as a measure.
GPM surveys are distributed by the crew (usually the CSD as part of their duties,) to supposedly customers randomly selected by seat number. These seat numbers are written on the envelope the surveys come in. In reality it is difficult to achieve this due to customer refusals, empty seats etc.,. As the CSD is measured by the percentage of returns they achieve, quite often customers are `selected` to fill in the surveys with the obvious one sided results.
As for individual comments,(letters to BA, comments cards etc.,) it seems these are rarely replied to apart from the standard replies.