Hi
Try these steps: (take a config backup in case you need to restore and start again)
From the web interface:
1) From Network setting menu click Broadband, find the VDSL interface, edit this, this should have your Broadband credentials here, scroll down and make sure IGMP Proxy Enable is ticked, should be by default, so either tick it then apply or cancel out if no changes.
2) From the same Broadband tab click 'Add New WAN interface', tick Active, name can just be BTTV, Type should be ADSL/VDSL over PTM, Mode should be Bridge, Encapsulation: PPPoE and IPv6/IPv4 mode: IPv4 only, under the VLAN section tick Active, 802.1p: is 0 and 802.1q should be 101, click Apply
3) Now from Network Setting again go to Home Networking, and under LAN setup make sure IGMP Snooping is enabled and IGMP Mode is Standard mode, save changes if necessary
4) From Network Setting menu go to IGMP/MLD, and put a tick in Membership Join Immediate (IPTV) and Apply changes.
Hopefully this will work, I've not tried it but taken settings from various docs about it.
Regards
Phil