What should i do?

Hello everyone, 

I have grown a Eucalyptus gunnii from seed, it's been planted for about 4 years so is about 18ft tall, its been hidden behind a shed, little did i notice that the bracing straps broke some time ago it seems, the tree has been rubbing really badly on the shed and other things, the bark has been rubbed away and exposed the core, at approx 1ft, 4ft and 7ft, woodlice have moved into 1 area, should i cut it down past all the damage, or just stop it from rubbing, clear the dead material and see what happens?

thanks