Just thought of another REALLY depressing piece, by a composer who I gather was (possibly still is, I can't remember whether I ever heard of him dying) a bit depressive: the Symphony No 7 by Allan Pettersson. I mean, this one is BLEAK. I'd have to say, Alistair, that it's a lot more depressing than anything in your 5tet.
And I remember a friend once describing some of Frank Bridge's piano music as 'music to slit your wrists by'.