When Kenneth Horne invited Julian and Sandy to sing at a Christmas party, they entertained the guests with campfire songs. Obviously.
“Lau your luppers on the strillers bona!” Sandy instructs Julian, which is Polari for, in literal terms, “place your fingers on the piano keys well”. That would be “play something nice on the piano”.
Vada: to look, or this case see: “Bona to vada your dolly old eek”
Mons star: if that is indeed what Sandy says, a Mons Star is a medal awarded to those who served with either the British or Indian Expeditionary Force in France or Belgium between August and November 1914. What other star he could be referring to would have to remain a guess …