Yes/No: Inside the Indyref (BBC Scotland, 10pm) ****

THOUGH it is five years on, the ability of Scotland’s independence referendum to start a fight in an empty house is undiminished. An ideal moment, then, for the new BBC Scotland channel to take a closer look at what went on during those heady days leading up to 18 September, 2014.

What a revealing look it proved to be. Filmmaker Paul Mitchell, whose previous work includes Inside Obama’s White House and The Death of Yugoslavia, used the first of three episodes to drill down from the top of the campaigns to the middle layers.

In doing so he unearthed some small but perfectly formed gems, including the moment Project Fear was born; how Jim Murphy wanted PM Gordon Brown to call a referendum in 2007 but was rebuffed; and, intriguingly, Nicola Sturgeon’s use of folders as part of her negotiating strategy. First, Mitchell had to set the scene with footage from the campaign. “For some it was fun,” said narrator Julie Graham in her best Procurator Fiscal on BBC1’s Shetland voice, “for others ugly”. Can’t argue with that.

Keith Bruce: I'd tell you what I think of new channel if I could find it!

Several of the talking heads would have been only too familiar to Scottish viewers. Then there was the next tier, the special advisers, some of them now departed their previous posts and therefore more at liberty to speak freely.

Among the obvious absences was David Cameron, who will have his say on the indyref and much else in his long-delayed autobiography. It did not matter much, because Mitchell’s team found someone better in Andrew Cooper. Cameron’s director of strategy from 2011-13 acknowledged that he had “never really been” to Scotland before he was asked to do in-depth polling.

BBC drops Nazi dog salute man

His research divided the Scottish electorate into defined groups. At one end were the pro-independence “Scottish exceptionalists” and “blue collar Bravehearts”; at the other “hard pressed Unionists” and “mature status quo”. Between the two were the undecideds. This was the ground on which the referendum would be won, or lost.

Of these undecideds, Cooper said: “More than three quarters of them said that their feelings about the referendum were summed up by the phrase, ‘My heart says I want independence for Scotland but my head says it may be too risky’. We concluded that with the time and resources we had available, the central thrust of the campaign strategy had to be to focus on the risks, because it was only the risks which were stopping them being Yes voters.”

To put it another way, this was the moment Project Fear was born. No-one said the phrase, but they did not need to.

One of the benefits of a long-form documentary is that the filmmaker and subjects spend a lot of time together, far more than would ever happen for a newspaper interview. Interviewees relax, open up more than usual. This was particularly the case with the First Minister, who revealed that she went to a crunch meeting with then Scottish Secretary Michael Moore carrying two large folders, only one of which had papers relevant to the meeting. Her aim: to give the impression she was “absolutely on top of things”. It was meant to intimidate him, and it did.

Behind the scenes at Central Station

(On another light note, the programme functioned as a guide, should you have needed one, to Ms Sturgeon’s changing hairstyles down the years, from the early, Juliette Binoche dark bob to the current fair crop.) The hour ended with the starting guns on the campaign about to be fired. If this first instalment is any guide, this series, which has one of the BBC’s finest, Allan Little, as a consultant, is going to be a must-see. Not the last word on the events of 2014, but enough to keep the fires of debate burning.

Available on BBC iPlayer