DRUMMONDS Bank sits on one corner of Trafalgar Square. Down one street is Buckingham Palace. On the other side is the Palace of Westminster. It’s about as close to the heart of the British state as you can get.

It was founded by a Scot, Andrew Drummond, in 1717, whose brother William was killed fighting for the Jacobites at the Battle of Culloden.

The National: The Battle of Culloden.

If you’re searching for an example to illustrate the ability of the Union to shapeshift and accommodate contradiction to its advantage, the story of Drummonds Bank is perfect.

It’s the story the BBC’s new documentary Union with David Olusoga reaches for when it wants to show how the Union was strengthened and formed more by realpolitik than ideology.

This is not really a documentary about Scotland, England, Wales and Ireland. It’s not really a story about the UK, either.

Its focus is sharper than that. It’s a story about the Union as a shifting, changeable and powerful force which has defined the histories of four different nations for centuries.

Olusoga is interested in how the Union came together, how it has survived and adapted over the years – and more than anything, the people who have been the subjects of these historical forces.

He manages to convey a great level of detail over four episodes, each an hour long.

There are genuinely shocking moments, achieved without recourse to cheesy gimmicks like re-enactments, including showing Guy Fawkes’s signature after he had suffered torture when he was caught attempting to assassinate James VI and I in protest over the Union of the Crowns.

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Olusoga manages to convey some of the worst episodes in the history of the Union – a particularly chilling moment comes as drone footage captures the ruins of an Irish workhouse as he describes how institutions like that were flooded by hundreds of thousands of starving people during the famine – without moralising or preaching.

The National: Ravenscraig steelworks in Motherwell

He also paints impressive portraits of the big moments in the Union’s history, such as the Easter Rising, while paying attention to smaller though still significant injuries as the closure of the Ravenscraig steelworks in Motherwell.

It is especially impressive that many of these stories are told at a human level, digging through archive primary source materials like censuses and contemporary records to pick out individual stories.

One of these comes in the form of Olusoga’s own family tree when he reveals he is partly descended from farm labourers from East Lothian. Olusoga – who hails from Newcastle – tells us the moving story of David Ewart, one of his ancestors, who returned to Scotland a broken man after the Napoleonic Wars.

He was effectively a child soldier, left the army illiterate and only able to sign his name with an X . At the end of his life, Ewart was buried in a pauper’s grave after a life working the land following his discharge on the grounds of “insanity” – what we would likely now call post-traumatic stress disorder.

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Portraits like this and others are painted with care and sympathy without sentimentality.

Olusoga has a historian’s head for the harsh realities of life while maintaining a humanity in his descriptions.

The last episode deals largely with the Irish independence movement and then the Troubles and does a fine job of portraying those conflicts without taking sides, though will leave disappointed those looking for meatier discussion on Scottish politics of the 20th century.

Despite the discussion of Ravenscraig as a microcosm of the types of issues which have fuelled the rise of Scottish nationalism as well as demands for devolution, the Scottish independence movement is slightly overlooked.

This could be because, to date, it has yielded no concrete historical results to speak of, so it is outsourced to talking heads from either side of the constitutional debate.

A deeper discussion of decades of Scottish constitutional wrangling may also have been an awkward fit with a show that functions as something of a “people’s history” of the Union.

But what it does tell us is the Union is a living, breathing, changing thing – it was never fixed and never will be.