I finally worked it out. The melody of Andy's GWC reminds me of the Christmas carol, O Christmas Tree/O Tannenbaum.

I kept hearing the part where the melody goes up but it has taken me a couple of days to finally identify the tune.

This is what I kept hearing in my head as a kind of melody break which isn't actually in Andy's GWC but the rest of the Tannenbaum melody seems similar to the Andy song - well, it does to me, anyway.

X:1
T:Mystery earworm
C:Unknown
M:6/8
K:D
L:1/4
Q:1/4=180
AAF c3|BBA A3|AAF d3|ccB B3|