If a thing is slap-bang on , it would mean "exactly right".
His answer to the quiz was slap-bang on .
That said, one would not usually refer to a person as being: slap-bang on.
Please note: the term is slap-bang [right, as in right in the middle of] and usually referring to a THING, not a PERSON.
Slap-bang is used to intensify: the dog was slap-bang in the middle of the road.
The other term: to be bang on means to be right.
slap-bang on merely makes it stronger.
So, if you want to join /bang on/
the verb to /slap-bang/
the adjective, the best way would be: slap-bang on .
That said, it is very odd to say that: Princess Diana was slap bang-on.
The only meaning here would be: She was right about the royal unspoken code of marriage.
That means: She knew that if Charles could have Camilla, then she Diana was perfectly justified (slap-bang on or right about) about having that lover of hers (I forget this name).
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"Slap-bang on" is a BrE colloquialism that means (more or less) "exactly", or "right".
For example: They tried to take the tube and landed slap-bang in the middle of the daily rush of commuters.
In this case it means that while Diana was branded by the media as "paranoid", she actually was correct that Prince Charles was romantically involved with Camilla.
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I wonder if this is a personal quote or a misquote because it isn't grammatical.
" They branded Diana, and still do, an hysterical, diva-ish and paranoid woman, but Diana wasn't paranoid about Camilla and the royal unspoken code of marriage, she was absolutely slap, bang on."
They branded and still brand Diana, as an hysterical, diva-ish and paranoid woman.
However , Diana wasn't paranoid about Camilla and in regards to the unspoken royal code of marriage, she was absolutely correct.
"Slap" and "Bang on" are slang expressions that simply mean exactly correct (at least in the opinion of the speaker/author).
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If you look at the definition in the Oxford English Dictionary, Slap , and look in the adverb section you can see definition 1.1.
1.1 Exactly; right: Princess Diana was correct in her suspicions about Camilla Parker Bowles.
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As a "Yank from across the Pond" it seems to me the UK slang "slap-bang" is akin to our own "dead-center" (fairly self-explanatory).
We also have the slang "dead-on" which means perfection .
The problem seems to be the conflation of "slap" with "bang on" being a somewhat odd combination as I read it.
Over here the closest thing to "slap-bang on" might be "dead-on center" in US slang.
This is not a very common usage but you may occasionally hear it none-the-less.
Both would mean "perfectly correct" in the final analysis.
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From  Grace Dent on Television: Harlots, Housewivs and Heroines - a 17th Century History for Girls, BBC4 | The Independent :   They branded Diana, and still do, an hysterical, diva-ish and paranoid woman, but Diana wasn't paranoid about Camilla and the royal unspoken code of marriage, she was absolutely  slap , bang on.     What does  slap  mean in this context?
What does "slap" mean in this context?
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<meaning-in-context><british-english><slang>
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"Slap-bang on" is a British English colloquialism , meaning "exactly right". Slap-bang is also used to intensify, to emphasize "exactly". If something is slap-bang on, it means it is "exactly right", "exactly correct". "Slap-bang" is used as an intensifier in British English. In US English, the expression "dead-center" is the synonym of "slap-bang". Here, it means that Diana was right in her suspicions that Camilla was Charles' lover. However, "slap-bang" usually refers to a thing and not a person, like in the example at hand.
If something is slap-bang on, it means it is "exactly right", "exactly correct". "Slap-bang" is used as an intensifier in British English. Here, it means that Diana was right in her suspicions that Camilla was Charles' lover. In US English, the expression "dead-center" is the synonym of "slap-bang". However, "slap-bang" usually refers to a thing and not a person, like in the example at hand.