From 1958, the Union Castle shipping line gradually reduced its service calls to the island. Curnow Shipping, based in Avonmouth, replaced the Union-Castle Line mailship service in 1977, using the RMS (Royal Mail Ship) St Helena.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Which shipping company replaced Union-Castle Line mailship service?
Curnow Shipping