This could be your last chance to demand a Final Say on Brexit. Do not miss it

If you have been on the previous Final Say marches or attended our rallies, you may well have seen the short film on great campaigns down the years narrated by the wonderful Scottish actor Brian Cox.

Despite having seen the film dozens of times when we were editing it with director Mark Lucas, and despite having seen the final version at events around the country, the hairs on the back of my neck still stand up every time I hear Brian deliver the final line:

“… Because everything is impossible… until we make it happen.”

It’s partly the voice; partly the delivery. But its real power comes from the setting of the People’s Vote in that broader story about making change through campaigns.

Here is the full script – and while of course the pictures that go with the words are a huge part of its strength, you can still feel the force without them:

“So many of the things we value… so many of the great steps forward we have taken… seemed impossible, until they happened.

“The NHS was called a pipe dream. The establishment voted against it 21 times. The fight went on. Until a final vote made it happen.

“Peace in Northern Ireland was a moonshot. It could never be. Until some brave leaders took a chance on peace. And people north and south voted for progress.

“Votes for women… civil partnerships… devolution for Scotland and Wales. The movements against fascism and apartheid – often opposed by the powerful. But made real by the people.

“We came and stood in the rain together. Marched together. Sang together.

“And now we fight again. For our families. For our communities. For our children.

“It is time for a People’s Vote on the Brexit deal.

“Because everything is impossible… until we make it happen.”

That final line was authored by none other than Nelson Mandela, who made me stand to attention whenever I was lucky enough to be in his presence. His words should be an inspiration to all who campaign for anything worthwhile.

And winning for the British people a final say on Brexit – not an election unlikely to solve anything, but a referendum to check whether, on the basis of what we now know, the country wants to proceed – is a fight worth fighting. And it is a fight those of us who oppose Brexit can win.

I know we’ve been told, over and over again, that it’ll never happen. But we were also told, over and over again, that we’d be out by 29 March. And we’re not; Theresa May is. Now we’ve been told, over and over again, that we’ll be out by 31 October. The prime minister (in name only) never tires of saying it. The cabinet likewise. The newspapers that make up the Brexit Lie Machine ventilate the message on an industrial scale. The £100m of public money “spaffed up a wall” telling us we are leaving on 31 October and that we are ready if we do – we’re not, and we’re not.

On a train from Preston to London last week, a woman said to me: “I’ll be on the big march… I’ve been on all of them, but I do wonder sometimes whether they have any effect.”

I have those moments of doubt too. How do you avoid them when you consider that the Brexit madness has delivered up Boris Johnson as prime minister at this crucial point in our history? How can you not feel dispirited at the lies, the incompetence, the sheer second-rateness of him and his team?

But it is at these moments that we need to take inspiration from Mandela, and from all campaigns of the past which, at so many points, felt impossible.

There is no way of quantifying which parts of the campaigns we featured in the film delivered the change. But in all of them, people power played a part. It’s why Mandela made a point, not long after his release from jail, of coming to London to thank all who had marched and petitioned and protested against the regime that held him captive. He would never meet most of them, nor know their names. But he recognised their role.

Marches have already played a part in getting us as far as we are. When we first started to agitate for a People’s Vote, we were likened to those Japanese PoWs who could never admit the war was over. Getting the mainstream media, or all but a handful of MPs, even to acknowledge that another referendum might be worth considering, was hard bordering on impossible.

“Will of the people, will of the people, will of the people…” But what if the will of the people changes? “It won’t. The people have spoken. Suck it up.”

It was a march in London in 2017, which drew a crowd of around 100,000 people, that led to a change in tone of media coverage, and emboldened more MPs to put their heads above the parapet. Marches organised at party conferences last year helped to shape the parties’ agenda.

Jeremy Corbyn may not have gone the whole hog road to Remain, but Labour has gone the whole hog road to a Final Say referendum. Some Tories who said they would never back a People’s Vote have done the same, however reluctantly.

Then there was the first really big march, last October, when not just tens but hundreds of thousands gathered in the Capital to make their voices heard – then an even bigger one in March.

From that day, our centrality to the debate was inarguable. EU leaders also realised that both May and Corbyn’s insistence that there was no demand for a new referendum was not accurate.

Campaigning is like being on a football team. You have ups and downs. You have periods when you’re on top, and periods when you’re not. You have times when things go according to plan, times when they don’t. You have unexpected advances and unexpected setbacks. You have star players who turn up and perform, others who don’t. You have squad players who emerge as stars. You also have the power of the crowd, some for you, some against.

It is partly as a response to the demands of the crowd that marches take place. The Together for a Final Say march on Saturday will be attended by many who have been asking themselves and others what they can do to help.

That is the cry of the committed, and indeed the desperate, down the years. And it is a feeling felt most intensely at times like now, when politics and the political process seem so weak and unable to adapt to the changing currents of public opinion.

The march provides a part of the answer. It provides a focus for the many and varied campaign groups that have sprung up since June 2016. Given it will cost a fair amount to stage, it might give those wealthy businesspeople constantly moaning about Brexit, and the damage it will do to their country and their businesses, the inspiration finally to get their chequebooks out and donate to the cause.

So far, we have raised what we need through crowdfunding – almost half a million quid. But the march’s success will depend less on money or on which politicians speak from the platform than how many people come, how much passion they muster and how much noise they make.

For all the talk of social unrest in the event of another referendum, I believe that if we win this fight, the country will ultimately rejoice. How, by contrast, will the country react if Johnson succeeds, and we are out by 31 October? Rejoice? I somehow doubt that our streets or our hearts are going to fill with joy as he drives the country over the edge of a cliff to plummet head-first into national decline.

This march is deliberately timed to follow the European Council at which Boris Johnson will try (he says) to get a deal, and its importance has been boosted by his decision to recall parliament for a rare Saturday sitting on the same day. It really will be a moment in history.

So if you’re not up for marching at a time like this, when would you be? Spread the word. Bring your friends. Bring your family. Don’t miss your chance, possibly your last chance, to make a difference.

Alastair Campbell is an adviser to the People’s Vote campaign