The Offshore Conspiracy — John Newton Chance

Ferne inherits an island off the coast of Cornwall which is occupied by squatters - Brett Bottray, gangland king and his formidable mother. When Ferne receives an invitation to a houseparty on the island, he accepts; not to demand restitution, but to scout about his island. The guests at the party are a mixed bunch; none of them seem to know why they are there. When the corpses start to pile up and an attempt is made on Ferne's own life, he realises there is a sinister reason for Bottray's occupation of the property.