This is perhaps a treasure, but not likely. The London bookseller I purchased it from stated that it was the 1st UK ed and the publisher in the book states, "first published in the UK 1955". After some research I have determined that an ed was published by Little Brown in 1954, although it states cover unknown. I strongly suspect that the Little Brown issue was the 1st UK edition 1954 and that this is a later reprint, not nessessarily 1955 but a little later. I wouldn't put it past 1960 given the age and format. If I had looked more closely I also would have seen that the publisher refers to "selected New Fiction" referring to other contemporary books.
It was highly unlikely to be the 1st UK edition. So I am embarressed that I didn't figure that out in the book store. How could I been so stupid! On the bright side, I didn't pay that much and I do not regret buying the book. I am going to read it. It is no longer in print and any copy of the book is scarce, it would seem. It is fiction and from the description I think it will be a fascinating read. It states, "The nightmarish horror let loose in ...compelling novel is the product of the social disease called "McCarthyism". Perhaps that is why this book somewhat slid into oblivion. I have read other books by this author and this one looks very good.
On the other hand, this is a US writer and perhaps the Little Brown HC in 1954 was the 1st American edition and not published in the UK till the Robert Hale edition but my instinct still thinks this is a reprint and there was an earlier UK ed. Oh, the angst of a collector!