It's a moment of pure chivalric triumph: as the English cavalry regiments advance across the Bannock burn, the knight Sir Henry de Bohun charges the Scottish king at full pelt. Robert the Bruce, mounted on a Highland pony and armed only with an axe, expertly ducks the knight's lance, and brings his blade down on De Bohun's helmet, slicing – as one chronicler puts it – "from crest to chin".

This weekend, an expected 15,000 visitors will watch re-enactments of some of the most celebrated and bloody combats at Bannockburn Live, a two-day family festival in Stirling marking the 700th anniversary of the battle of 1314, when Robert the Bruce of Scotland successfully defended his crown against the English army of Edward II.

More than 200 warriors from Clanranald, famed for their work on Hollywood blockbusters such as Gladiator and Thor, will condense 48 hours of battle into 30 minutes, along with music from Julie Fowlis, King Creosote and Karine Polwart, a clan village and food and drink stalls.

The Battle of Bannockburn is embedded deep in the collective imagination of Scotland, commemorated to this day in the unofficial national anthems Scots Wha Hae and Flower of Scotland – the latter was only written in 1965. Like most historic events, it has been overlaid with dubious myth and convenient embellishments as the centuries have progressed, but it remains at heart a story about the triumph of ordinary Scottish people over perfidious English elites, with Bruce the embodiment of the nation: tenacious, strategic, undaunted by the oppressors' might and ultimately leading his country to freedom. With less than three months to go until the referendum on Scottish independence on 18 September, the resonances are hard to avoid.

The year has been so crowded with international events – the Commonwealth Games, the Ryder Cup, the second year of the vast cultural history festival Homecoming, even the MTV Europe awards – that one might almost think it had been planned that way.

But while other events showcase Scotland as a cosmopolitan, multicultural, confident country, any celebration of Bannockburn inevitably risks falling out of step. Indeed, tickets sales for Bannockburn Live have been slow, though Visit Scotland remains confident it will reach its break-even target on the day. The commemoration was once a highlight of the SNP calendar, but gone are the days when Alex Salmond quoted Mel Gibson's Braveheart at his party conference. The party is all too aware of the dangers of being seen to foster a romantic, backward-looking ethnic nationalism that runs counter to the egalitarian, inclusive vision of independence that it hopes will win out in September.

The political choreography is particularly challenging. This same weekend, Stirling becomes only the second Scottish city to host the Armed Forces Day national event, which is taking place about a mile from the Bannockburn festivities. David Cameron and the Princess Royal will join thousands of serving personnel and visitors for a veterans' parade, masses, bands and aerial displays from the Red Devils and RAF Falcons.

Some pro-union politicians have expressed concern that the Bannockburn anniversary will garner support for the yes campaign, with the Tory former prime minister Sir John Major last week suggesting that the SNP chose 2014 for the referendum in order to exploit "anti-English sentiment". He accused the Scottish government of allowing this commemoration to overshadow the year's other military anniversaries.

But the author Louise Welsh, whose Radio 3 documentary on Bannockburn was aired last Sunday, says: "History is endlessly useful to parties and political campaigns, but the SNP have taken a conscious step back [from Bannockburn] this year to avoid seeming jingoistic. In the past, Bannockburn has been used in many different ways. In the Victorian era, it was used to foster the idea of empire; it was later used in the opposite way by home rulers and then nationalists."

For Welsh, the appeal of Bannockburn and Bruce is axiomatic: "We like the story: the underdog winning against a much stronger force, the story of valiance and generalship."

"It's no small thing to stand against a highly trained horseback army holding a big pointed stick," she adds, referring to how the Scots, outnumbered three to one by the far better equipped English cavalry, moved in "schiltron" formation, advancing on foot in huge circles bristling with wooden spears.

Welsh also notes, though, that the idea of Bannockburn as a straightforward fight for independence against English rule is simplistic. "Robert the Bruce had killed John Comyn, who many people thought was the rightful king of Scotland. He was brave, but he was also out for Bruce. Scots fought on both sides of the battle, and there was a civil war ongoing in Scotland before they fought the English." Fergus Ewing, the Scottish government's minister for energy, enterprise and tourism, rejects Major's charge of anglophobia.

"We need to understand and celebrate our history," he says, "and this applies to all our history, whether that's 1314 or 1914. The idea that someone goes to see a battle re-enactment then goes out and votes yes [in the referendum] is tenuous at best.

"I think that those who assert there's an overt and crass political motive here are wholly out of touch and wrong."

Last Saturday, in sharp contrast to the mass Bannockburn rallies in the 1980s, a modest gathering of local SNP activists and clansfolk laid wreaths at the Borestone, where Bruce is said to have planted his standard the night before the battle. Speeches focused on the contribution of the "sma' folk", whose intervention on the second day of the battle proved decisive. Streaming down from Gillies Hill, armed with little more than pitchforks, Bruce's camp followers were mistaken for fresh Scottish troops by the combat-weary English and overpowered them.

Joseph Graham, laying a saltire of flowers on behalf of his clan, drew a direct link between this people's army and the grassroots foot-soldiers campaigning for a yes vote. "This vote means that now the people have a voice. The majority in Scotland are the 'sma' folk' and we will determine our own destiny."

His ancestor John de Graham fought alongside William Wallace in the first war of Scottish independence, which culminated in the battle of Bannockburn. Now 66, Graham has been coming to Bannockburn annually for 50 years. Three generations of his family are there, including his granddaughter Leigh, who says: "Something like this commemorates our history. It's not about living in the past. We're looking to the future now but you can't do that without knowing where you've been. When I was young this was like a festival, with music and face-painting. It's sad we're not allowed to celebrate our history without it getting buried under politics."

Angus Macfadyen played Robert the Bruce in Mel Gibson's Oscar-winning film, which has just been rereleased on DVD, and which he credits with swinging a yes vote for devolution in 1997. "Twenty years ago everybody wept in the cinema and then went out and voted for a Scottish parliament. That film had such an influence on the cultural landscape."

He is sympathetic to why campaigners may wish to see less emphasis on the romantic nationalism associated with Bannockburn in this referendum year. "I don't think that the Scots like to be manipulated, so they are wise to stand back from it." The Bruce story resonates, he says, "because it's our story".

"The truth of some elements of it have been debated, for example the spider story." Legend has it that Bruce's contemplation of a spider failing six times to attach its web but succeeding on the seventh attempt gave him the courage to confront the English at Bannockburn. "But people connect to the pure stubbornness of Bruce, never giving up."