Just back from taking my class: Colin Matthews' Night Music is a piece that continues to give me pleasure, although most of his work leaves me cold. Having said that, closer acquaintance with Aftertones (written for Huddersfield Choral Society, later performed by the Halle and recorded by them by the BBC) reversed my opinion of it being a piece of retro nostalgia; I think now that it does have something significant to say.

Recent Birtwistle - OK, there's nothing so gut-wrenchingly powerful as the Triumph of Time, but what we get is still music of substance (I'm thinking of The Last Supper in particular), which I think is where PMD fails.

Absolutely (there are several posters here who have played Hymnos, methinks). Have you ever played the little-known Clarinet Sonata?

Any composer 'tinkering' with a major composer's uncompleted works must surely have had the thought of it being performed flitting across his mind at least once...

As for the Holst, well I'm possibly made of baser stuff than Colin Matthews, but if I were a composer I'd be thrilled at the prospect that my name would be associated with one of the most popular pieces in the repertoire, and concerts would finish with my music even though the majority of the audience had no idea who I was or vaguest interest in what I'd composed or go on to compose...

That's assuming that I accepted the commission in the first place, and assuming I'd be OK with Imogen Holst spinning in her grave... 

OK, but Anthony Payne obvious has a real passion for the piece (and not just for the money) that comes across very strongly when he talks about it. There is more at work than chasing money.
I don't doubt that CM was thrilled at the prospect of this commission and at the potential royalties, but if I was commissioned to write Pluto, I probably would have done so as well...
I don't, and never have liked composer subbuteo, where we set up our team against the opponents and try to score goals against them (or better yet, flick them off the table).
I hope I don't come across as being cross... I was actually incredibly placid when I wrote that. For once in my life I wasn't being sarcastic!
Seriously though, I'm interested in everybody's definition of a "proper composer", just so long as we understand that the definition may well differ from person to person and that this is not necessarily a bad thing.

Although editing a piece for performance is completely different from deciding a completed work needs another movement. Matthews himself declared that Imogen Holst would probably have been horrified.

I see that several posters here have stated that much of the more recent "work" of PMD and Birtwistle leaves them cold or words to that effect. What does this say about the "composers" concerned? Are they "musicians" in Robert Simpson's sense? Is it possible for a Musician-composer to be such and yet still to compose rubbish? I find that I need only listen to a few seconds of something contemporary to know whether it is any good (for me) or not. So to those who have been asking "do I know this or that work" usually the answer will be "no" because it didn't survive the 5-seconds test. In the "other place" under a different name I mentioned once that I have three categories of music that one suddenly chances on: 1) something I want to hear to the end, and again many times (eg Bruckner, Bach, Allan Pettersson; 2) something I don't mind hearing to the end of, but don't want to hear again (no example possible because the music made insufficient impact to be remembered); 3) quasi-music which I don't even want to hear to the end of (i.e. most contemporary music, especially PMD, Birtwistle, Turnage, and many many more). You may think that I am some sort of ignoramus to be saying this, but do bear in mind how little art from any era actually survives through time once its creator is dead and has no leverage through publishers, broadcasters, pocket critics etc. to go on being performed, and can only survive because common or garden music lovers and listeners actually want to hear it (provided their music establishment is structured so as to permit it). Also, I am a composer, and studied composition professionally for 3 years at a major music institution in London with a composer now dead but reasonably well-regarded. So I think that I should not simply be dismissed as a crank with no ear for post-romantic music. The truth is that so much of contemporary music (and other art-forms) is total rubbish, existing for reasons of egotism, money, fame etc. and saying nothing in substance. It is, in fact, an exercise in the "Emperor's New Clothes" tradition. But as I said in another post: if you can really derive pleasure from listening to PMD et al. then good luck to you: you have more sources of enjoyment than I do.

t-p, Brava!