"We'll gather lilacs" is in a song called, er, "We'll Gather Lilacs". My mum used to occasionally warble it. From a 1945 musical "Perchance To Dream" according to Wikipedia. The article also refers to Ivor's final effort, "Gay's The Word".

See also Robert Graves' "Goodbye To All That" for a hilarious anecdote about Novello "keeping the home fires burning" during the Great War.

I only recently twigged that Patricia Routledge had a successful career in musical comedy long before her TV stardom, so it's not surprising that she could pull esoteric lyrics from 1907 out of her hat!