Ive just been reading SIN 498 which states that
Where the CP is using PPP and intends to use Multicast for GEA, the modem shall be able to detect and process multicast frames differently
to unicast. Multicast for GEA frames sent into Openreach (IGMP reports) shall not be encapsulated with PPP otherwise they will passed
transparently as normal GEA traffic.
Seeing as the VR900 is automatically configured for BT and encapsulated as PPPoE this will likely be why its not working atm and a new WAN interface needs to be added. According to Openreach
here IGMP Multicast is encapsulated as IPoE which is why it will be working out of the box for TT. So I think you somehow need to set up another WAN interface for IPoE and bridge them.
I cant immediately see in the GUI how you'd do this, as I dont want to mess with my own settings at this late hour.
As Im out early tomorrow morning and not around until late in the evening, I hope you dont mind but Ive passed your address on so that he can contact you direct.