serious

Root canal fillings : expensive things.

Not, however, painful ones. In my case. Or finished ones. One more appointment, although apparently this was the tricky (and costly) one. I now have bits of (AFAICT) silver (!) wire embedded in my jaw. I do wonder if I picked this up right.
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.
Why have you been expecting a duet? There aren't that many bits where it's the siren singing.

Glad to hear your root canal work is going ok.
You know you're in trouble when a dentist says "You might feel a slight twinge here".
Cheers. Another totally painless experience - even without any anaesthetic this time.

The song's been floating around for over 35 years. I'd have thought somebody would have tried it by now.
You'll like it if you like them - they're a band of great consistency

That's the best veiled insult I've heard in weeks :)
In their defence, they actually do have a much nicer sound than Status Quo do. I thought the drumming was a bit ragged at times, though.
(looks around room under lowered eyebrows)

Squirms uncomfortably. I've never heard of the song and I've never heard any songs by This Mortal Coil.
In this case, This Mortal Coil sounded very like the Cocteau Twins, because what you could hear was largely played by the Cocteau Twins. It was Ivo Watts-Russel's musical project. Not being a performer himself, but running a very respected music label (4AD) with some terrific people on the roster, he rounded them and various others up and directed them through three albums. Really rather good, as long as you like stuff in an eighties 4AD style.
This Mortal Coil's version also features rather heavily in the David Lynch film, Lost Highway which I found rather offputting.

In your defense, I foolishly thought that one was the original : /

PS Zotz - what brought all this on in the first place???!
they're a band of great consistency

That's the nicest way of saying "all their songs sound the same" I've heard yet...
More or less based on the description of one of the later Devo albums as being less than an artistic endeavour than an exercise in quality control.