A mystery shopper from within the organisation cannot be effective. When they encounter a problem they will understand all the things behind it and will not react the same way as a regular Pax.
There are many companies who specialise in Mystery Shopping and charge an approiate fee. To have them do all the routes once a year would be far more cost effective than taking a perm member of staff from their regular job and having them do something for which they are not qualified.
Not qualified? No. If EZY want to know how their service is seen then they need to hire the right people. In my view, this is a short sighted waste of money and an internal HR/
PR disaster. It will back fire on them. In fact, it probably already has.