M. P. Shiel and the British author of The Ingo tetralogy,  a series of four children's novels set in Cornwall, are not both of West Indian descent.
Helen Dunmore FRSL (12 December 1952  5 June 2017) was a British poet, novelist and children's writer. 
Matthew Phipps Shiell (21 July 1865  17 February 1947)  known as M. P. Shiel  was a prolific British writer of West Indian descent. 
The Ingo tetralogy is a series of four children's novels, set in Cornwall, by British author Helen Dunmore. 
