The vast majority of mystery shoppers are employed on a casual basis. Simply put 'mystery shopper' into Google and you'll find dozens of firms offering to put you on their lists. Here's just one of them:
http://www.mystery-shoppers.co.uk/becoming_a_m ystery_shopper.htm
Most firms don't employ their own, full-time, mystery shoppers because it doesn't take long before someone recognises them and their presence is no longer a mystery!
Only very big organisations normally have in-house teams. One of these is the Association of Train Operating Companies. They have staff who travel around the country, taking notes on the standards of service provided. Unlike most mystery shoppers (who usually try to stay inconspicuous), the ATOC people often deliberately act out the r�le of the awkward customer to see what response they get. Some individual train operating companies also employ their own staff for discreetly monitoring staff performance. So, it might be worth writing to ATOC or your local train company.
Chris
PS: If you do get a job as a mystery shopper, it helps if you don't tell too many people about it. When I was running a railway station, I was once approached by a woman who asked me if she park her car for free for a while. She explained that she was a mystery shopper, checking up on the W H Smith bookstall. I was happy to write out a parkin permit for her. Then, while she went to park her car, I had a quick word with the manager of the bookstall! By the time she got back, the floor was swept, the staff had all put their badges on and the all of magazines were lined up with military precision ;-)