To find out the current time source:
w32tm /query /source
To resync a server or to force a server to update it’s time with the source:
w32tm /resync
Setting external time server:
w32tm /config /manualpeerlist:pool.ntp.org /syncfromflags:MANUAL Stop-Service w32time Start-Service w32time
A List of Time Servers: KB262860
How to configure an authoritative time server in Windows Server: KB816042
UK Time Server Pool can be found here.