The place to get info about the evidence and arguments used in court is the 21st century edition of The Severed Alliance. It goes something like...
In English law, if you join a band you are assumed to be in an equal partnership with everyone else in the band, an entitled to an equal share of the money it makes (not including songwriting royalties, which belong to the songwriters, not the band), unless you agree otherwise.
So, the case was about whether Morrissey and Marr could show that Joyce had at some point agreed not to be an equal partner. The key occasion when this was claimed to have happened was in October 1983 at Pluto Studios. Morrissey wasn't present, but Marr acted for both of them, telling Andy and Mike that they had to accept a reduction in their stake or he would quit the band.
The first problem with this is that Johnny's ultimatum might be considered coercion, so that it could not result in Andy and Mike agreeing to anything legally binding. But, even if we say that it wasn't coercion, the main problem is that it's extremely unclear that any agreement was reached. Andy and Mike testified that there was no agreement. Johnny testified that there must have been an agreement, because he wouldn't have just let it slide, but couldn't remember what the agreement exactly was. Importantly, no-one, including Johnny, could recall specific percentages being discussed. So, at best, we have a discussion which was inconclusive and ought to have been picked up at a later date.
Later, in 1984, all four Smiths plus Scott Piering went to a solicitors' offices, where Mike and Andy were offered 25% each if they agreed to cease being partners five years after the group split (i.e. after that point they would get nothing). They turned the offer down, and Morrissey said in court that the offer never had his agreement anyway. But the thing about the existence of the offer is that it obviously undermines M&M's claim that the thing had been sorted out at Pluto studios.
Later still, perhaps sometime in 1985, the amounts of money being set aside for Andy and Mike in the Smiths' accounts was reduced to reflect a drop in their share from 25% to 10%. This doesn't seem to be related to any new discussion, but just something Morrissey and Marr had decided to do. Rourke and Joyce are either unaware of this or choose not to make an issue if it until after the Smiths split up.
Nobody puts anything relating to an agreement down in writing at any point.