Hello Stu H,
What is the exact version of Symantec Drive Encryption are you installing. Symantec Drive Encryption 10.3.0 and previous did not support computers using UEFI firmware and would run into this issue, therefore, this issue could occur if computers are running UEFI firmware without emulated BIOS.
Please go through the following article -
http://www.symantec.com/docs/TECH180744