An artificial form of tourette syndrome in which the singing or speaking of a line from a song by one individual produces the spontaneous and not entirely voluntary singing or speaking of further lines of said song by another individual. This phenomenon is most often seen among people in good moods.

"allow it" means to just leave something alone, or don't worry about it. People in West London say this quite often.

being unable to control the temptation of suddenly bursting into song

To randomly break out in song