Thank you for the comments.
I can confirm that the engine it a defender 200 tdi so top mounted turbo. See attached a picture of the set up running (pulled air box off as leaking oil but crankcase pipe feeds into main body of the cartridge type filter housing).
Interesting comment about excessive crankcase breather pressure; when it first happened I removed the crankcase pressure pipe and started it up and visually you could see oil droplets in the air flow but I do not know if this is normal. If there is excessive crankcase pressure what would be the potential root causes?
The engine overrun happened very quickly, I did de-clutch and turned off the ignition however did not react to stall the engine, but it stopped on its own after about 5-7 seconds (assumed that it just limited to the oil in the intake). I’m hoping that as it started again to pull on to the drive that is a good sign that I have not done too much damage!
I am going to explore further today, but the oil drain does not appear to be kinked.
In terms of turbo replacements, I have seen that you can buy just an internal cartridge, has anyone had any experience swapping the main components and do you require any specialist tools?
Thank you