I was in contact with the archive of the Otava publishing house, who put out the book that left me astounded some days back: Brian McDermott's The Battles, called Tuntematon tilaa näytännön in Finnish. Here's the original post on the subject.
So, the Otava publishing house still has the contract made with Brian McDermott. Unfortunately the contract didn't tell if McDermott's book was already published in book form in English, but the contract retained the possibility that the Finnish publisher could also use it as a serial in a film magazine. And a friend of mine happened to have the said magazines at hand, and - ta-taa! - Brian McDermott's story was indeed published as a serial in a magazine called Elokuva-aitta (Film Storehouse or some such in translation) in 1966-1967. This leaves the question: was McDermott's story published as a serial also in English? I don't seem to be able to find any trace on this possibility, though, but it's still a viable explanation.
However, there's also a possibility that McDermott wrote the story exclusively for the Finnish audience. It was said in the magazine that McDermott was a popular figure in Finnish television due to the BBC language program called Walter and Connie in which he performed. It was also said that the characters in the novel resemble the Walter and Connie of the show, so it's possible McDermott based his characters on them. (I still haven't had the chance to read the book, so can't comment on that.)