It is still not entirely clear to me, but the "dandy" is definitely
Eberlin, the central character, who is some kind of double agent,
living in London:
[Eberlin] lives in a posh flat, wears elegant clothes and dines
in the best pubs.
Maurice Rapf, reviewing the film, Life 1968/03/29
And we have a comment from Derek Marlowe, author of the novel,
who surely originated the phrase:
Marlowe assumed a classical definition of the dandy for his story,
explaining that it is not simply about dress but really about
self-discipline, a discipline that denies friends, sex, and ostentation.
His character of Eberlin, he explains, "retires into his own entity,
a dandy in aspic, untarnished".
Alan Burton, Historical Dictionary of British Spy Fiction