Jesus wept this thread is full of idiots (Thom and Bellend excepted - SHOCKER)
Extracted the remains of the plug and replace it with a new or good second hand one, they didn't receive it with a broken inspection plug so why hand it back with one.
Oh right - replace things that are well past their sell by date because why?
The plug will have been brittle and old. If they were truly conscientious then they should have picked up the phone when it happened and explained what had happened and that you were in for buying a replacement plug, if you wanted to, but it wasn't necessary.
But was this at a top-dollar workshop?
Was it even at a VW specialist who is more likely to care about your knackered old car?
I doubt it somehow.
Your old car is there to have a job done. If bits of your old car break when they're doing the job, well maybe they should tell you, but maybe they figured it's an old car and you're just there because you want a job done cheaply.
If you're going to start throwing your weight around at least know what it is that they've broken, to avoid looking like a complete fool.

Take it back for a discussion about the uneven running pulley, as the four securing bolts might not be tightened up properly and it'll knacker the ribbed and V belts as it waves around.