Days without number - Robert Goddard

Michael Paleologus, retired archaeologist and supposed descendant of the last Emperors of Byzantium, lives alone in a remote house on the Cornish bank of the River Tamar.
A ridiculously generous offer has been made for the house, but he refuses to sell despite the urgings of his children, for whom the proceeds would solve a variety of problems.
But the stalemate is soon tragically broken.
Only then do Nick and his siblings discover why their father was bound at all costs to reject the offer.
Their desperate efforts to conceal the truth drag them into a deadly conflict with an unseen and unknown enemy.
Nick realizes that the only way to escape from the trap their persecutor has set for them is to hunt him down...