Director Richard Loncraine about FULL CIRCLE after first screening in Cannes Market, May 1977:
"I wasn't so interested in the horror aspect of the story, what appealed to me was the chance to make a film in which there were two parallel stories, capable on entirely different interpretations. Curiously, nearly all the women who see the film think it is a ghost story, while men think it is the story of a breakdown."
there are some SPOILERS ahead:
there were hints that this (the film) is a subjective pov of julia. So that Mirrors are important, maybe showing that she is reflecting herself.

SPOILERS!!!
Yes mostly a subjective POV of Julia...the mirrors, an obvious visual symbol of split personality/schizophrenia...first time Julia sees herself in the mirror brought by Mark, she is scared of her own reflection/guilt, also think of the early scene in the bathroom when she washes the blood of her wounded leg, she sees her own reflection/guilt in the mirror, then she cries and says (to her dead daughter we presume) "My baby, forgive me" ...
then at the end, when she stares at her reflection in the bathroom mirror (just before Olivia the "ghost" child appears in the mirror) Julia isn't scared anymore of her own image...maybe she has accepted her guilt, or madness...her fate?
One of the fascinating things about the ending scene is that we don't really know what Julia thinks sitting in that chair...is she really aware of what's happening?
The director Richard Loncraine said about the ending that it had a double way of understanding it, it's up to the viewer:
either Olivia the "ghost" was Julia's delusion and Julia kills herself with her daughter's sharp toy, or Julia accepts to be killed by the evil child/her daughter look-alike (if you believe there is a ghost)...but in both cases like Loncraine said "Julia finds her child and her solace."

there were these hints of conspiracy, or you felt that some of the protagonists were playing wrong
Yes at least in the first half, with Magnus's character and Lily's ambiguous feelings about Julia:
she seems to genuinely like Julia but she follows her brother's dodgy idea to set a seance in the house, to scare a bit more Julia and to have her money.
But also the cursed storyline was very interesting and really a bit scary
Indeed!
First time i watched the movie i found many scenes/shots to be scary:
the scenes with Julia alone in her big house (the "Don't be frightened!" bit for example), Magnus's unwelcome intrusion in the house/the basement scene, the sharp toy in itself is a bit sinister, the "dream" (??) scene with the hands, the seance scene, the genius unexpected shot during the mental home scene of Olivia's eyes "piercing" through Julia's face (or is it a delusion of the half crazy old mother thinking her dead daughter came back from the grave?)...
and the stunning ending of course, i find this great picture (you could make a poster of it, really) of Olivia silently looking at Julia and then moving towards her still very creepy to this day! Even the scene where Swift the alcoholic man is talking about his nasty childhood is kind of creepy, sinister...only because of the great lines and low key acting.
"Low key" in fact seems the key word in that movie...the acting and the quite subtle way it creates fear and dread, mostly offscreen without jump scares and blood.
There is this one scene, where Julia lies in bed on the right side of the frame in a dark room and on the left side the closed door shows a line of light underneath and Julia is sitting up and talking clearly and loudly into the dark: "don't be afraid!" and she is saying that TO someone. i think that shows that another person is talking right now - talking to julia, she is talking to herself, she is talking to the imagined ghost.
But is is not clear. The accidents that happen seem to be by chance, bad luck, a curse, or is Julia the curse?
That's the idea. There are questions but at the end no answers. You understand what you want to take of the film's ambiguous mystery/story:
is Julia's house really haunted? or is she talking to herself, in fact, during this scene?
Director Loncraine said that in this particular scene he wanted to "reverse" some of the clichés of the haunted house stories:
here in this scene and in the whole story the lead character isn't especially afraid of the "haunting"...because of her loneliness, despair, she WELCOMES a child , she welcomes the "ghost"...even when she learns that the child is a nasty, killer little blonde girl...
There was only one thing that threw me out of the movie and made me wonder too much and it's at the very, very end so it didn't hurt that much;-) and maybe i'm too specific about it, but here is one change of tune in the song while the camera moves away from julia, where the theme keeps repeating and then it kind of "has" to change, it felt like they felt:" damn, we cant keep this on longer" and the tune goes up and thereby changes the mood slightly and it just threw me off. yeah, silly i know;-)
No it's not silly, i can see what you mean, the last track "changes" twice but i think it's precise editing, first to slowly reveal Julia's "change" (her death, when that great tracking shot around the chair reveal her deadly neck wounds) then when the end credits start:
https://www.youtube.com/watch?v=CyumkItcZiYI remember first time i watched the film in the mid 90's i thought that the film was going to end on the lovely, sweet cute shot of Julia accepting the "angelic" Olivia in her arms, that the film was going to end on the camera doing a half circle around the chair, "Stay with me...stay" and then fade to black, end credits...it would have been a beautiful poetic ending, already.
BUT it ended with the lights going out, the camera endlessly moving around the chair and slowly revealing the character's lonely, tragic fate (in my opinion the whole story is about her slow suicide because of overwhelming guilt and loss)...i was like OH MY GOD!! jaw dropping moment!...incredibly unsettling, moving but somewhat beautiful/darkly poetic.
The score was very good, sometimes on the edge of too cheesy, because at first it just drags you with it, so emotional, but when you're into the mood of the film, it works very well.
"Emotional" is the word! It sounds mavbe a bit "dated" now especially the 70's keyboards sound but that soundtrack is something...it really nails the sad, bleak, erratic wandering moods of Julia's character and the story...very pretty melodies and a big part of the film's atmosphere with the visuals and Mia Farrow's "ghost-like" performance. Loncraine said that he and Mia Farrow mostly agreed to make the movie when they listened to a demo track Colin Towns recorded in pre production before the shooting, Loncraine said "i could see a movie in that track".
Some of my favourite scenes/shots enhanced by the soundtrack are the opening credits scene, Julia's encounter with her new house, pictures of Kate all around the floor, the walk in the park, the car drive at night under the rain and the whole ending.
There is this one scene, where Julia lies in bed on the right side of the frame in a dark room and on the left side the closed door shows a line of light underneath
The frame compositions, the use of widescreen cinematography is really beautiful in that movie...all the scenes in the house for example, Loncraine said that he used Panavision, Cinemascope to make the house (and in that scene the room) like a character...Julia on the right side of frame, the closed door on the left side, the space is used skillfully in many compositions. A claustrophobic feel at times (lots of close up of Julia's face, showing us her feelings, without dialogues/lines).