I took my car in for a routine annual service at a local garage. During the service, a piece of debris from a broken spark plug fell into the engine. The garage says it was 'just one of those things' that could not be prevented, and I have to pay for the cost of repair (including sending the head away to a specialist). Is this right - ie is it really the case that reasonable care on their part could not have prevented this happening?
Usually, reputable garages would in these kinds of circumstances pay for the accident. However, reputable garages have a higher labor rate than non-reputable garages, so you get what you pay for. I would first try to negotiate with the garage, then find out about the consumer protection laws in your country if that doesn't help.
About sending the head to a specialist: I don't believe there's any reason to do that if there's no reason to suspect the head is bad. I presume the idea is that if the head is removed from the engine, it's a good opportunity to test if it is good, but considering you didn't require a typical repair of a damaged head gasket, there should be no reason to suspect the head is bad. How much did the specialist service cost, and what percentage it was of the final price? I probably would have omitted this specialist step if all that happened was just a broken piece of spark plug in the engine.
Not an answer per se; but in similar situations I've had great luck putting the contaminated cylinder to BDC by hand, filling up the cylinder with foamy shaving cream, and then adapting a shop vac nozzle to fit over the spark plug hole.
Somebody holds the shop vac in place while somebody else cranks the cylinder back to TDC slowly by hand - mindful of any binding.
I have a borescope to inspect the success, but if you didn't you can sift through the vacuum canister to verify the offending contamination was removed.
My "answer" would consist of no, that's perfectly avoidable by any competent mechanic. And certainly should have been rectified at no cost to the customer. A proper spark plug socket would not let any insulator pieces fall out past the socket and into the cylinder.