I think he has played loads of different basses over the years and I've seen different ones every time I've seen him play. I remember seeing him playing a Rickenbacker once. He does tend to play the Shergold for "lead lines" rather than actual bass parts, but there have been songs which have needed a rounder bottom end and I presume he picks the bass for the job/song. "Transmission" has a typical Rickenbacker sound, for example.
I do agree that "Twenty Four Hours" is the classic track and a great example of a song built completely around the bass line. When this first came out in England every bass player was trying to get this sound and incorporate into their songs - I wrote at least 2 or 3 that were blatant rip-offs of this song!
Much later, I came to realise that the biggest influence on the "sound" of Joy Division was the late Martin Hannett and if you check out any album that he produced around this time you will hear what I mean.(cf. Magazines's "Correct Use of Soap")
Apparently in the sessions for "Closer" Hook (and the rest of the band) were constantly arguing with Hannett to get the sound more raucous and aggressive. Hook himself said in an interview with Q magazine : "I wasn't satisfied with it at the time, but now I think it's the most beautiful, melancholy thing I've ever done."
I think the most important thing that Hannett did was give credibility and a sense of meaning to the whole thing. He found ways to bring out the character of Ian's voice and Hooky's bass style. He even managed to make the synth stuff sound elegant 
. It's too bad though that both he and Ian died - such a waste of talent. I even enjoyed how he treated New Order's first album. Sometimes when I'm in a mood I'll put the first minute of 'Dreams never End' on repeat 
