So after a bit of investigating I ahve this fixed so thought id post the answer here in case others have the problem.
Go to https://www.asus.com/uk/Notebooks/ASUS-ZenBook-Pro-UX501VW/HelpDesk_Download/ and under Utilities get ASUS Thunderbolt firmware Update NVM_v15_TI_v1.5
Once the firmware was updated for thunderbolt it worked fine, I’m getting approx 400MB/sec on copying alarge 20gb file.
