Sorry, late to the party.
I don't really know a lot about cars. They have wheels, right ? (though, RTKM, jazzybee is a car nut - though again, more about latest models etc. than mechanical stuff)
Anyway, ripley: 200 miles for anything sounds like utter crap.
If they fitted some exhuast putty to fix a hole, sometimes that needs to be baked. They do that with the car just sitting there and the engine running.
If they fitted new pipes / exhaust boxes / catalyst I can understand the need to check the installation. This would still probably be done stationary or by a SHORT journey of a few miles.
Basically, it sounds like a member of staff borrowed it, quite possibly without asking, had fun in town, possibly a shag / grope (button) and then brought it back. The garage has obviously had this sort of problem in the past as the lady on the phone didn't try and dispute this with you.
If this was the garage that claimed they did work to remedy the AA check in the beginning, and then didn't, I'm afraid you're going to a crap garage. There are plenty about.
Try and get petrol / fine / valet / money for the aggro and for not going to watchdog, and don't go there again.
HTH