It seems widely accepted that the Romulans are decended from opponents of Surak and his teachings and that they left Vulcan during the Time of Awakening but I can't remember this actually being out right stated in any episode.
Is this just some form of fanon or have I missed an important reference some where.
Your memory serves you well from a strictly TOS point of view, there was no such reference in the original series and in my not humble opinion the conclusions drawn from the available information for the later Star Trek are erroneous.
Balance of Terror:
STILES: We know what they look like.
SPOCK: Yes, indeed we do, Mister Stiles. And if Romulans are an offshoot of my Vulcan blood, and I think this likely, then attack becomes even more imperative.
MCCOY: War is never imperative, Mister Spock.
SPOCK: It is for them, Doctor. Vulcan, like Earth, had its aggressive colonising period. Savage, even by Earth standards. And if Romulans retain this martial philosophy, then weakness is something we dare not show.
While we learn that Vulcan apparently had an aggressive (space) colonizing period, Spock has no specific knowledge or suggestion whatsoever regarding Romulan offshoots. The "if" could suggest he didn't expect Vulcan descendants that far out.
The Savage Curtain:
SURAK: The image of Surak read in your face what is in your mind, Spock.
SPOCK: As I turned and my eyes beheld you, I displayed emotion. I beg forgiveness.
SURAK: The cause was more than sufficient. Let us speak no further of it. In my time, we knew not of Earth men. I am pleased to see that we have differences. May we together become greater than the sum of both of us.
SURAK: In my time on Vulcan, we also faced these same alternatives. We'd suffered devastating wars which nearly destroyed our planet. Another was about to begin. We were torn. But out of our suffering some of us found the discipline to act. We sent emissaries to our opponents to propose peace. The first were killed, but others followed. Ultimately we achieved peace, which has lasted since then.
"Devastating Wars" sounds like a reasonable explanation that many records of Vulcan history prior
to the advent of Surak had gotten lost.
This is a good example of TOS continuity where we finally learn of possible reasons why Spock simply couldn't deliver more information about the origin of the Romulans in "Balance of Terror".
"The Savage Curtain" clearly reveals that Vulcan history had been recorded and preserved, after
Surak's arrival, otherwise Spock couldn't have remembered such details.
But apparently, the exodus of those Vulcans that did not agree to Surak's philosophy was not recorded to come up as an option for explanation during "Balance of Terror"?
The other big questions are
- why did the exodus Vulcans embark voluntarily on a difficult journey (presumably nuBSG style?) to the farthest reaches of space ?
- why did the exodus Vulcans voluntarily deprive themselves of warp drive technology so that a few thousand years later all they have is "simple impulse drive" (regarding cloaking technology it's not impossible they got access to this advanced technology accidentally from a crashed ship or the like from the b&w inhabitants of Cheron)
I think that the name "Romulans" and its obvious allusions to ancient Rome might have been a hint. What did the Romans do with aristocratic VIPs they didn't dare to execute? They exiled them to the farthest corners of the Roman Empire.
In this analogy the Romulans might have simply been considered to be the offspring of exiles, relocated to the far reaches of space and with no realistic possibility of ever returning to Vulcan given the interstellar distance and lack of warp drive knowledge, all prior to the advent of Surak.
Of course, retcon activity in TNG and further deprived us of such a possibility but from a strictly TOS point of view, I believe that was possibly the backstory intended.