this sounds like it might be a fungal infection . I wouldnt put another leylandii in the same place. scrape the bark and if there is a white mushroom smelling layer then it may be honey fungus. tere doesnt tend to be acure for this sadly.

If you have to replace it please do not plant another leylandii. They do need careful looking after and while you may have the time the next owner may not. They grow at a terrifying rate of 1.5' every year.