Is it Julia or Jennifer?Now I think it might be Ann Shapland, instead.
Right a new one;
"....and being a very obstinate, as well as a very stupid man, he refused to admit he'd made a bloomer."
is it non-series novel? - Yes it is.
Is It Miss Marple in Caribbean Mystery talking about Jason Raphiel? - No it is not.
Is it one of her late books? - Yes.
Let's say after 1960s? -Yes, it's after 1960, but not later than the decade of the 60s.
Hope that makes sense.