Dave, I'm sure your right about the spurious mention of St Helena but it did catch my eye because of a man who is buried in Walkley Cemetery.
Cornelius Valentine Clifford is described on his gravestone as 'A native of St Helena, Africa'. He and a brother both served for a few years in the Royal Navy and his brother is described on a statement of his services as 'a man of colour'. When their father was baptised on St Helena in 1844 he was described as an 'emancipated African'.
While researching these people I discovered a lot of parish registers for the island at the University of Witwatersrand.