I've got a theory about this movie
I think this is one of the best movies of the 1980s. I have not seen the original or any other of Dennis Potter's work. I'm in the middle of "The Singing Detective" right now, and I dislike it. I would be finishing it, but the second disc is still in the mail. (I'm watching it through Netflix).
I've found that Potter fans are harshly divided on this movie. Half include it with "Singing Detective" and the original miniseries as Potter-esque work and the other think that it totally destroyed the tone of the original.
My theory is that the miniseries likes its characters. The movie loathes them. (Much like it is with the not-great-but-profoundly-misunderstood "Buffy the Vampire Slayer" film). I think that its difficult for people to grasp onto the idea of a movie that loathes its characters. Potter's TV work was like an angry howl. There was something behind the crying. Not here, it's too black to even be pessimistic.
The movie is shallower then what I have seen of "Singing Detective" but it's also profoundly more challenging and experentially terrifying.
It's funny, the two styles are so different that your preference is likely to be flavored by the one that you saw first.