Sarafo - I'll try and answer this to the best of my knowledge:

Alec John Such was their original bass player from 1983 to 1994. Around the "Keep The Faith" record, it is said that Alec really wasn't holding his own in the band anymore. He was making several mistakes in concert that was throwing the band off. During an interview, Alec told the reporter that Jon had been giving him grief about it. Jon had Alec over his house and fired him.

Hugh had been a friend and studio player of the band for many years (in fact he's credited on the first album.) However, as Jon has said, the band has been together so long that they never wanted Hugh as on official band member, even though he technically has been their bass player now for 10 years. Personally, I think it's kind of silly at this point, but that's another story.

Some of that may be a tad bit out of order, but for the most part, that's the story.