I used to work for Citroen UK.
This isn’t meant to sound harsh, but this IS how Citroen UK warranty will look at it.
It should have been serviced every 12.5k or 2 years. You’ve serviced it late, the failure of the turbo can easily be attributed to poor oil quality due to late servicing. Therefore it’s not a manufacturing defect so not warranty.
FYI the engine will most probably be scrap too, the oil way’s will be clogged with burnt/old oil/sludge. This is normally the cause of the turbo failure – poor oil flow.
There is a procedure to repeatedly flush the engine to try to clear the sludge. It’s a circa 10 hour job on top of the turbo replacement. If you replace the turbo without having this done, at very best you have a 50% chance of the turbo lasting more than 3k miles.
Again sorry if this sounds blunt just trying to give you the ugly truth.
Mike