I tried the FritzBox 7490 with the latest firmware and it didn't help the situation. I know that it says is does work with Multicast, but not in the same way that BT Openreach have implemented it.
In order to get it to work, you can use the settings above and try and create a new virtual interface that is not encapsulated in the same way.
I can confirm that with some messing around, I managed to download the tool, and get it to use the English files for the interface. Once I had that, I then implemented the changes above and multicast channels all work.
So it is possible, but it certainly isn't clean.
It would be much nicer if Fritz made the change at their end for BT customers, but I have asked them and they were not that responsive.