Hi,
If you locate the Domain Controller configured for PDC Emulator FSMO role and run the commands below;
net stop w32time
w32tm /config /manualpeerlist:0.uk.pool.ntp.org,0x8, /syncfromflags:manual /update
net start w32time
w32tm /resync
Also ensure that outbound NTP (UDP port 123) is enabled on your firewall.
All computers that are members of that domain are configured by default to synchronize from this DC and you should also be able to use this as an NTP server itself that Sophos Web Filtering box points to.
Hope this helps.
Regards
Mark
Technical Solutions & Pre-Sales Manager
Ergo Computing