With regard to Song To the Siren, my finding so far are as follows:
This Mortal Coil did one of the definitive versions, which I assume you all know (looks around room under lowered eyebrows). Big Bob Plant's version's not bad (according to the version I've heard) but not incredibly memorable either. The version by the Czars is actually rather good, and worth a listen if you get the chance. Relatively sparse, controlled and quite straightforwardly musical. Dead Can Dance don't seem to have done a version, although Brendan Perry has and it's pretty much exactly what you'd expect from him. Also, therefore, worth hearing. Incubus Succubus haven't fone a version, but have a song called Song <EM>of</EM> the Siren. You'll like it if you like them - they're a band of great consistency. True Faith are a trance-influenced pop outfit if their version's anything to go by. Snuff and HMHB do what you'd expect, although HMHB's is less distorted. Amusing but not amazing. Cul de Sac weren't bad, but I preferred the Czars.
None of these were duets. I am slightly surprised at this. I'd have thought somebody would.