After growing these a few times, I know that they require a lot more water than box, though even box would struggle in the planting conditions described by the OP. Sorry! gardening is a bit live and learn. But it's good practice to start with a small plant of a species you're not familiar with. Garden centres get lots of money off these very large specimens, and everyone wants them for instant impact. The garden centres don't care if they die! But the buyer does. It's a scam really. Your plant isn't dead but it will probably have a lot of dieback. However, don't cut anything off until you've followed the watering advice above. With a larger specimen like this, too, it will need a lot of generous watering until it has managed to root into the surrounding soil. This need for lots of water will go on for at least a year after planting.
Wait until it begins to put on new leaves on the apparently 'dead' parts (perhaps as much as the spring after next). Only then will you be able to see what is really dead, and only then should you prune it back.
Sorry for not posting. I think we haven’t watered it enough so will give it more water this year. To soil is sandy and well draining so lack of water seems the issue.
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Wait until it begins to put on new leaves on the apparently 'dead' parts (perhaps as much as the spring after next). Only then will you be able to see what is really dead, and only then should you prune it back.