Publisher:Mayflower

Storypilot says:
 * British subsidiary (Mayflower Books Ltd) of Dell Publishing Co, Inc, formed in 1962
 * Acquired by Granada Group Ltd in 1967
 * Transferred to Granada Publishing Ltd in 1968

Some stated addresses:
 * 1969-1971: Mayflower Paperbacks are published by Mayflower Books, 3 Upper James Street, London W.1
 * 1972: Published by Mayflower Books, 3 Upper James Street, London W1R 4BP - reprint same year mentions Granada Publishing Limited
 * 1973: Granada Publishing Limited, Frogmore, St Albans, Herts AL2 2NF

An interesting site for the Horror books is Morbid Mayflowers.