I believe I do have a timeline of events that you might find informative.
Gally phones Newt to ask him to do her a “colossal” favour by filling in for her at a Midnight & Noone performance in Leeds. She tells him that he will need to play the cello, but will only have one small bit of singing in the Albert Small song.
Despite Newt’s protests, Gally insists that he must do this for her. Gally tells him that he can borrow her clothes, but he will need to buy himself a top hat. (Presumably, this is because his head would be too large for Gally’s top hat.)
Newt is worried that the audience will be disappointed if they are expecting a male impersonator but get an actual man. Gally dismisses this by saying that they will just think he is frightfully good.