Install Hyper-V role fails on Windows 2016 during server restart

..with error: "we couldn't complete the features" during the windows boot  and then mentions "Hyper-V" service superficially, stops the service and then removes the service. Can boot back into the server after 2 restarts, but hyper-V not available. 

I've followed a number of guides. Here's a few bits that are relevant:

Hardware: Dell T20 with updated BIOS + Virtualisation enabled in bios. 

OS: Windows Server 2016 Standard

"CoreInfo -v" produces:
Intel(R) Xeon(R) CPU E3-1225 v3 @ 3.20GHz
Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
Microcode signature: 00000025
HYPERVISOR      -       Hypervisor is present
VMX             *       Supports Intel hardware-assisted virtualization
EPT             *       Supports Intel extended page tables (SLAT)

SysInfo:

Hyper-V - VM Monitor Mode ExtensionsYes

Hyper-V - Second Level Address Translation ExtensionsYes
Hyper-V - Virtualization Enabled in Firmware Yes
Hyper-V - Data Execution ProtectionYes

Only one Network adapter (hardware card) listed in Device manager - updated driver. 

I did install Docker prior to Hyper-V - not sure if that will effect things. 

When WS2016 restarts, the Server Manager shows an error "request to add [features] failed" and advised I peruse the event viewer - however, I have absolutely no idea where to look for that!]

Hope someone can help, because baffled by this!

thanks