For me, firmware v14 doesn't work with multicast TV. I've reported this to Zyxel support and am waiting a reply, I'll report back when I get a answer.
Update:
It's a week since I first reported this issue to Zyxel support, I've just chased it up and got the reply.
"this case has been escalated to our headquarters"
Update2:
Still no sensible reply from Xyxel support but I have discovered the following telnet command restores multicast TV to my youview box when using firmware (14C0).
interfaceGroup delete group_name ConcurrentWAN
This command is not required with other firmware versions.