I don't know if this helps, but have you contacted your local trading standards authority? They may know of organisations which contact them to report problems, and these organisations might employ mystery shoppers. You could also get in touch with Which? magazine, I am sure they use mystery shoppers.
Hi, I live in N.I. and work as a mystery shopper. I'd visit www.gapbuster.com or the NOP group who both provide jobs in N.Ireland. The amount of work you get depends on where you live or how far you will travel. If you live in a city you'll get plenty of work, you'll not make a fortune but it's nice getting paid to shop and getting free stuff. Hope this helps and good luck.