Did the garage also change the oil feed and return pipes at the same time as the turbo ?

A lot of turbo failures are caused by a lack of lubrication - basically, over time, oil carbonises in the feed/return pipes, leading to those pipes getting clogged up. Much like arteries and a heart attack, the lack of flow, or a 'lump' getting dislodged and making it's way to the heart'turbo, then causes the failure.

Unfortunately, you've made the classic error of 'nreaking the chain of liability'. Always get a garage to supply AND fit parts. That way, if something goes wrong, you've got one port of call, and not 2 different people, each potentially blaming the other.

The only way to prove the liability of either party would be to get an independent engineers report showing exactly what went wrong, and why it went wrong.

Did the garage also change the oil feed and return pipes at the same time as the turbo ?

An extremely pertinent question. I'll bet they didn't. Schoolboy error.

Oil supply problems are probably about the biggest cause of turbo failure. Dirt ingress to the compressor is another biggy.

Also take a look at my turbo care guide in the stickies. There's a high probability of "pilot error" making a contribution.