On the eve of his execution in 1915, the Swedish-American trade union organiser Joe Hill had a surprisingly upbeat message to pass to Bill Haywood, a fellow Industrial Workers of the World activist: "Don't waste any time in mourning. Organise." It is a statement Tony Benn and Bob Crow would surely have endorsed after a week that leaves the left grieving for two of its few remaining political giants. Both figures were demonised in life and are being patronised in death for the same reason – to dismiss any challenge to the status quo. The passing of an era; we will never see their like again: national sweethearts stripped of any political content who we can all admire – here is the gist of obituaries that suggest (or hope) that the left has died with them both.

But, as socialists, both Benn and Crow believed that social change did not hinge on individuals, however much they could inspire and provide direction. Their politics were based on a sense that human progress is driven by collective action, and that struggles from below force those with wealth and power to offer concessions. The left, they would be telling us now, goes on without them.

Although allies, Tony Benn and Bob Crow represented two sides of a divide on the left that has prevailed for well over a century. Benn was a long-standing parliamentarian who belonged to the Labourite tradition, believing that a party founded by the trade unions to represent working people was at least a potential vehicle for social change. Crow, on the other hand, was an ex-Communist who led a union expelled from Labour a decade ago; he believed in the need for a new party to offer workers a political voice, and focused on strike action to improve the conditions of his members. But both served as examples that the left is not as rigid and sectarian as it is often portrayed: Benn was a keen supporter of extra-parliamentary action who happily worked with nearly anyone on the left; Crow's union had a parliamentary group, exclusively made up of Labour MPs.

Yet their deaths will fuel anxiety among Britain's fragmented left. There is little escaping the fact that the left has been reeling ever since the high tide of Thatcherism. There are a host of reasons why: the rise of the new right; the battering of the trade unions, the traditional backbone of the left; the shift from an industrial working class to a more atomised service sector working class; a form of globalisation that limits an elected government's room for manoeuvre; and the way the collapse of Soviet totalitarianism was spun to suggest it was "the End of History" and free-market capitalism had triumphed for ever. Some thought the crisis of capitalism that began in 2008 would provoke a renaissance on the left, but it is the neoliberal right that has thrived.

The 2011 TUC March for the Alternative in London, protesting against government spending cuts. Photograph: John Stillwell/PA

So where is the left, and what is it? The first place to start is the British public. An audacious claim, to be sure, but even some rightwingers recognise that, on many economic issues, the electorate is to the left of the Labour leadership, let alone the political establishment. "Slowly but surely, the public is turning its back on the free market economy and re-embracing an atavistic version of socialism," wrote Allister Heath, the editor of London business daily City AM, late last year, even suggesting that free-marketeers faced being "annihilated". It was in response to what he called a "terrifying" YouGov poll commissioned by the union-backed thinktank Class, which suggested there was mass support for renationalisation of public utilities such as energy and rail, and for rent controls. Other polls show the electorate want taxes hiked on the rich and and a council house building programme. The inevitable retort is that many of these voters also want a clampdown on immigration and benefits, but the polls at least point to a huge potential pool of support for the left – if it can get its act together.

Labour's political complexion is worlds away from the high-point of Bennism in the early 1980s, but Ed Miliband owed his election as Labour leader to a promise to draw a line under Blairite orthodoxy. Labour's leadership has repeatedly disappointed and even angered the left, whether by backing the public sector pay freeze, accepting George Osborne's spending plans in the first year of a Labour government, or just not yelling loudly enough about the crueller consequences of austerity. But, unlike Tony Blair, Miliband has not tried to define himself against a wider left, thus preserving party unity and giving more radical voices space. And at the last election, a surprising number of leftwing figures were elected as Labour MPs, including charismatic former miners' leader Ian Lavery, Grahame Morris and Ian Mearns. Jon Trickett, a thoughtful Yorkshireman anxious for the next Labour government to be as transformative as Thatcher's administrations, sits in the shadow cabinet. And shadow ministers such as Wigan's Lisa Nandy give heart to those wanting a Labour party with an unmistakable progressive bent.

Shadow ministers such as Wigan’s Lisa Nandy (above) give heart to those wanting a Labour party with an unmistakable progressive bent, writes Owen Jones. Photograph: Sarah Lee for the Guardian

The more traditional Labour left, though, is in clear disarray. The Socialist Campaign Group, founded by Benn to co-ordinate socialist MPs and led by the indomitable John McDonnell, is a weak echo of what it once was. Left-leaning sentiments in the Labour party are largely unorganised, and the "soft left" campaigning group Compass abandoned its Labour focus after Miliband's election.

The biggest reservoir of leftwing activity outside the Labour party is the trade unions. Although they have halved in membership since Thatcher's ascendancy, they remain the country's biggest democratic movement, with a rank-and-file of around six million. Despite a hostile environment, they even grew in size last year. Unite and its Scouser general secretary Len McCluskey are the most influential, and demand a shift in direction from Labour as well as unapologetically supporting strikes and civil disobedience. It has led to the Tories and mainstream media painting them as bogeymen. The TUC's new general secretary, Frances O'Grady, is a refreshing break from former office-holders, who tended to be rather technocratic men in line for knighthoods. Since the Cameron-Clegg love-in, most large-scale dissent on the streets has been driven by unions. The union-organised March for the Alternative demonstration in 2011 was the biggest workers' protest for generations, and in the same year they co-ordinated the largest wave of industrial action since the 1926 general strike.

In an attempt to overcome the left's Judean People's Front-style sectarianism and build a broad anti-austerity movement, the unions launched the People's Assembly with a 4,000-strong rally last June. Huge meetings have been held across the country – I've spoken at several – but successfully chipping away at the austerity consensus remains a daunting challenge.

It is the Greens who have flourished most as a leftwing alternative to the Labour party, helped by the election of charismatic, street-fighting politician Caroline Lucas in Brighton, as well as MEPs and scores of councillors. But their ambition of becoming a leftwing equivalent of Ukip has yet to be realised, and a bitter dispute between Green-run Brighton council and bin collectors tarnished their brand. The Respect party, on the other hand, is in reality little more than a personal vehicle for George Galloway, who punched the political establishment in the nose after his shock election in Bradford's byelection in 2012. But his past praise for dictators and appalling comments about rape following allegations against Julian Assange have left him largely isolated. North of the border, leftwing, pro-independence movements such as the Radical Independence Campaign are agitating for a new progressive Scotland, free of neoliberal dogma.

The Trotskyist left, meanwhile, is in tatters. The Socialist Workers' party and their newspaper sellers have long been a recognisable, and – to other activists – rather irritating presence at leftwing events. For many years, they were Britain's largest far-left group, serving as the backbone for the Stop the War movement and its two million-strong demonstration against the Iraq war, as well as for groups such as Unite Against Fascism. But following allegations that the SWP leadership covered up rape claims, it has been deserted by many of its activists and is now a near-pariah on the left. Their largest rival is the Socialist Party, the successor to the Militant Tendency, which infiltrated Labour in the 1970s and 1980s. Its project to form a "new workers' party", however, has little to boast except a run of catastrophic election results and lost deposits.

Groups that reject the idea of “leaders”, such as UK Uncut (above) have thrived. Photograph: Antonio Olmos for the Guardian

On the other hand, groups that reject the idea of "leaders" have thrived. Following the coalition's assumption of power, UKUncut orchestrated occupations of businesses and banks in protest at tax avoidance worth £25bn a year. They even received favourable coverage in the Daily Mail, and helped force tax avoidance on to the agenda. When activists set up tents outside St Paul's Cathedral in 2011 as part of the global Occupy movement, they helped popularise the sense of a wealthy "1%" thriving while the "99%" suffered. Student protests and occupations in 2010 helped throw up a new generation of anarchists, hostile to both the established left and traditional trade unions.

What must surely drive all shades of the left is a sense of necessity. The sixth richest country on earth now has half a million people dependent on food banks; wages haven't fallen for so long since the Victorian era; the next generation faces being poorer for the first time in a century. From the Chartists to the anti-poll tax movement, there is a long tradition of causes facing apparently insurmountable odds, but being vindicated in time. "The flame of anger against injustice and the flame of hope that you can build a better world" is what drives social change, Tony Benn said: modern Britain does not lack anger, but the left's real mission is surely hope. Charismatic and inspiring leaders will inevitably be mourned. But the injustices that drove them don't die, and so neither will the need to continue their fight.