Yeah, it sneaks up on you, doesn't it?
Once again you can credit/blame SC "It has three settings!" Moll for the naughty innuendo with which "Castrovalva" is larded. I'm usually a little too Victorian to reach for these veiled references (or a little to hamhanded to make them veiled enough to be funny). Although, as Captain of this little Reliant-class vessel I am perfectly willing to take credit--er, responsibility for all that happens under my command...
I tend to think, actually, that "Castrovalva" and "Dalek," both by SCMoll, are among the best fivers at 5MDW.