NTP not working with firewall set on high

[#1]

Hi all,
I'm using opendns with the firewall up high, but my ntp stops working, is this just because of the firewall, or because i'm using opendns with a static setup ?.
i don't see any option to open port 123 from the plusnet web interface. Will adding a rule in my modem firewall to allow ntp fix the problem, i kind of think of the plusnet firewall and my router firewall as separate items not shared.
opendns family
208.67.222.123
208.67.220.123

[#2]

Pretty sure that is the plusnet firewall.
I recall seeing elsewhere some time ago that this is a known issue with firewalls 'set too high'

[#3]

From https://portal.plus.net/support/security/firewalls/broadbandfirewall.shtml

Quote

"Note: The 'High' setting will prevent the use of VPN software, Active FTP sessions and software which lets you to synchronise your computer's time using an external source. If you want to use these applications, switch to the 'Low' setting."

So it looks like plusnet firewall is causing the problem.

[#4]

Thankyou both for your help.
@npr i missed that small detail. "software which lets you to synchronise your computer's time"