I am making a presumption here but I would have thought that any death in prisons, natural or otherwise, would be reported to the prison's HQ and a record held in the prisoner's file (obviously there must be one) so would those records continue to be in existence and, if so, where would they be held?

Possibly a letter to the Director of Prisons could be the first port-of-call in any enquiry.

Another thought which has just occurred; the next-of-kin (or some other member of the prisoner's family) would have been informed, possibly regarding burial arrangements, so someone in the writer's family must possess more information.

What about medical records as the cause of death would have been established by a physician.