The case was not (officially, at least) about slavery, but Olaudah Equiano managed to get anti-slavery activist, Grenville Sharp involved and publicised what happened, to support the abolitionist cause. It's likely that they also influenced the legal case put by the insurers, which argued against the treatment of people as chattels.
It's not a straight historical play, there are modern references, elements of the story which are acted out and other's 'shown' as flash backs, and elements where direct transcripts from the court case were used.
The play is, I believe, to be fully staged next year, but this performance was a dramatic read through. And it was extraordinary - incredibly powerful and moving, and still, 230 years after the events depicted, profoundly shocking.
I wasn't able to stay for the post show discussion,but will definitely be planning to see the show when it is fully staged .