Didn't need to look it up. Engraved on my memory.
You could start by looking here:
Nobody knew how for sure much ash was coming out of the volcano
Nobody knew for sure how it was dispersing
Nobody knew for sure how it was dissipating
Nobody knew for sure how and in what concentrations it affected the engines.
So for each of these unknowns, a hefty safety margin was applied, which accumulated into the totally over-the-top decisions which were taken.
The world is divided into regions controlled by Volcanic Ash Advisory Centres. The London VACC has responsibility for UK, Ireland and Iceland (that's all ...). Because it happened on their patch, they called all the shots, without having to draw on expertise from elsewhere, including places which actually have experience of flying around active volcanoes, like Italy...
At one stage, the Toulouse VACC, which looks after the rest of Europe, Africa and the Middle East, overrode the London advisories and reopened French airspace because they believed there was no threat ...
An utter balls-up, from start to finish.