I went back to check in the book (Michael and Natasha) for any references to Poland and it does make one reference which states that - 'In Poland, Michael had owned land, estimated by the 1930's to be worth £600,000'. After Michaels murder, his wife was left without means of support and she made an unsuccessful attempt to claim his assets outside of Bolshevik Russia including this estate in Poland, which, I presume to be Ostrov.
Good Luck with the research.