wouldn't someone like Trading Standards be interested in being served something other than lamb when you order lamb....?
Sadly, it happens all the time and has been going on for decades as far as I am aware by Restaruants of all types, trying to cut costs. Recently, we had some issues with some T/A's using 'condemned' meat, One such T/A a few years ago that I had never used before, ordered the Chicken Biryani and the overriding taste was Paraffin, which is used to try to prevent the meat returning to the food chain. (Takeaway meal returned, refunded and reported, now closed).
We seem to have a growing number of 'meat processing' units on little industrial estates that can never be inspected because they are unregistered. Some use dubious practices to try to circumvent the law, such as relabelling meat and chicken with expiry dates based on the date it was delivered to the butcher/meat packaging firm and not the date it was slaughtered on, like the recently shut down firm that had a meat processing factory in Exeter.
There have been many documentaries made about Trading Standards ordering meals described as Lamb but ending up being any but, a little like the Horse Meat scandal a few years ago and unfortunately, since there is no equivalent of the Advertising Standards Agency, other things too, such as 'Hawaiian' Pizza - Ham and Pineapple but the Ham is not pork but Turkey Ham although the word Turkey is excluded.