Green expectations: Is the party finally breaking through in England and Wales?

As Bristol's newest Green councillor was hoisted, slightly awkwardly, on to the shoulders of his colleagues, the smiles could hardly have been wider.

Within hours of Patrick McAllister's by-election victory, the party's local Twitter account had been renamed to celebrate becoming "Bristol's biggest party" - they now have the most seats on the city council, although it remains Labour-run.

It is the latest in a series of local election successes for the Green Party of England and Wales.

The party has never managed to convert its grassroots support into general election success - with the exception of Caroline Lucas in Brighton Pavilion, who remains its only MP.

Unlike some other parties, the Greens have never gone in much for expectation management, and they are bullish about their prospects now.

"We think we could well get another 100 seats," says co-leader Carla Denyer, when asked about May's local elections.

At the moment, they have roughly 540 councillors in England and Wales, around 200 of them won in 2019, when these seats were last contested, and frustration at Brexit negotiations hurt both the Conservatives and Labour. They have run councils before but have never won outright control of a major local authority at the ballot box.

Ms Denyer says the growth of climate change as a political issue is "definitely a factor" in the party's recent success at the ballot and adds: "But I think that it's also true that people get a Green councillor in their local area and they like what they see."

Former MEP Molly Scott Cato, who's a member of the Green party's governing body, says one of the major reasons voters don't back the Greens is that they simply don't think they can win.

"Our job is to convince them that we would offer something better, that we can be elected and when we exercise power, we exercise it well and in their interests and without any suggestion of self-interest or corruption.

"I think we're doing that at local level and our job now is to translate that into trust at a general election so that we get more MPs in Parliament."

Caroline Lucas was elected as the UK's first Green MP in 2010 but the party has struggled ever since to secure a second parliamentary seat.

The party's biggest challenge remains the first-past-the-post electoral system used at general elections.

Their sister party in Scotland, where a form of proportional representation operates, has been in government at Holyrood, in a power-sharing deal with the SNP since 2021.

The Greens are polling at around 5% nationally, which would not deliver any MPs at all if it was repeated evenly across the country.

Like all parties, they have to target their resources at certain seats. At the next general election, widely expected next year, they will be focusing on areas such as North Herefordshire and Waveney Valley in Suffolk, where they have built up a concentration of councillors.

But it is in Bristol where the party sees their best hope for a second seat at Westminster, and where Carla Denyer will be taking on Labour incumbent Thangam Debbonaire.

Boundary changes are set to turn Bristol West into Bristol Central and make it, they say, "very Green-leaning".

It would still be a tough ask though. In 2019, Ms Denyer got 25% of the vote, but Ms Debbonaire won with more than 60%.

Local political experts also describe voters in Bristol as a "sophisticated electorate", whose votes at council level may not necessarily translate into support at a general election.

And, of course, the Greens have been optimistic before.

In 2015, the so-called "green surge" saw thousands of new members join up but it didn't translate into another seat in Parliament.

Natalie Bennett, now sitting in the Lords for the Greens as Baroness Bennett of Manor Castle, was party leader at the time.

"On our biggest day in 2015, 3,000 people joined the party in one day and we had local parties that went from five people to 200 people.

"We just didn't have the resources to direct all that energy in a focused, going to definitely convert to votes, kind of way.

"Before we were just leaping high with nothing to hold us up, now we've got the foundations."

Professor Stephen Fisher, an elections expert based at Oxford University, says there is certainly opportunity for the Greens to promote policies that would appeal to voters who liked Labour's policies under former leader Jeremy Corbyn.

"Labour's more cautious approach to tax and spend... makes it easier for the Greens to make more bold claims about how they would do more to save the NHS and invest more in public services.

"Greens might be able to get away with offering more spending on things that people like."

He warns though that it remains hard for the Greens "to get the momentum to be seen as major contenders" and Labour's decision to put Green issues at the centre of its pitch to voters makes it harder still.

Green councillors and vote share in general elections, 2010-2019

Across Great Britain as a whole, the Greens have approximately 575 councillors.

Of these, about 540 are in England and Wales.

The Greens' share of the vote across England, Wales and Scotland in general elections, and number of parliamentary candidates fielded:

Sources: Rallings & Thrasher/BBC

One issue that does set them apart from Labour - as well as the Tories and the Lib Dems - is Brexit.

The party backs re-joining the EU "as soon as the political conditions are right".

Adrian Ramsay, who co-leads the party with Ms Denyer, claims their larger rivals are offering "nowhere near enough" on climate change and the cost of living, and don't recognise "the scale or the urgency" of these issues.

The Greens' flagship policy is a wealth tax on the richest households with proceeds going to fund a massive programme of home insulation.

They remain committed to taking energy and water companies into public ownership, introducing a universal basic income, and have called for social care to be made free at the point of use.

But like the rest of his party colleagues, Mr Ramsay is hoping success at a local level will finally convince voters at a general election that casting a ballot for the Greens is a "normal option".