Alan is bitter because he feels he "wrote"parts of some of those DS songs, most notably Walk of Life (3 chords in it!).
I would say that his most important contribution is in Telegraph road, globaly the whol LOG album, live arrangement for Romeo & Juliet, and live re-arangements for "old" songs like once upon a time, down to the waterline, angel of mercy, in the gallery...but not in walk of life !
Yes, actually as a musical director, most of the long versions of the DS songs were co-composed and arranged by Alan, but never credited. As Guy uses to say, you are paying not only to play but also to add your parts to the song, and it must be credited to the author of the song anyway, as it written by him (MK) even you added parts yourself that were not there at the beguining.
I´m sure the 96ers do it everytime they play with MK, but the difference is that they know they are proffesional paid to do that. Alan, as most of the people in DS, felt they were a band, part of the band, but they weren't. In the end they got to produce and being credited in the OES album (the famous "produced by DS, DS are: Mark, John, Alan, Guy).