Tora,
The Labour Party say Corbyn visited several functions that evening.
If the video is the Labour Christmas Party (impossible to verify from that clip, it could be any party any time), then judging from the drunken singing, it was quite late. So they probably sang it when Corbyn was long gone.
I don't think there can be a ban on the song. People in the Party have been murdering it for donkey's years.
Given that Corbyn's enemies are denying there is any truth in the report, I reckon it is largely made up or exaggerated.