mid the post-election noise of Tory gloating and Labour skirmishing, the writ for another byelection dropped into news silence on Wednesday. Why should anyone care about Chesham and Amersham, a “safe” seat in the “blue wall”, vacant following the death of the well-liked Tory MP, Dame Cheryl Gillan. Move on, nothing to see here, she won 55.4% at the last general election. Yet this could prove a far more important test of all the parties than it looks. For the Lib Dems, on just 26.3% here in 2019, it road-tests whether they retain their old ability to snatch astonishing byelection victories from Tory heartlands. For Labour, trailing here on 12.9% last time, it tests its common sense: is it willing to work with other parties against the common foe, and stand right back or not stand at all? For the Greens, on just 5.5% last time, this HS2 route feels fertile ground, but if they fight here they risk taking the blame. Can the other parties reach out to them? “They’d better come to us with a cast iron quid pro quo,” says the Green party spokesperson Molly Scott Cato. Why would the Tories mind losing one from their 80-seat majority? It would deliver quite a body blow to a lot of MPs in similar seats, a warning that while Boris Johnson is off bribing Labour northern seats, his neglected back door is flapping open. Lib Dem polling and door-knocking in the seat suggests why the Tories deserve to lose: 55% here are remainers, many of them Tories who only stayed loyal for fear of Jeremy Corbyn. Brexit is still the great definer, and remainer Tories were alarmed by the hard Brexit deal. Voters here know this byelection could force a U-turn on one key policy they hate, the new planning laws in the Queen’s speech. The district council failed to make a local plan, leaving the area extra vulnerable to a developers’ free-for-all in the green belt. Many Tory MPs in similar seats, including Theresa May, made powerful speeches against it in the Commons this week, with widespread condemnation from the likes of the CPRE, the countryside charity, and others warning of a “rural sprawl” and a “dark age of development”, with no affordable housing. HS2 still arouses fierce passions, the Lib Dem local candidate strongly opposed. Out of 70 sample doors knocked by the Lib Dem head of campaigns, all but one voter claimed they’d never met a Tory canvasser: there are many such taken-for-granted Tory “safe” seats, as neglected as Labour’s proved to be. Many fear “levelling up” means pork-barrelling the north while ignoring southern discomforts. The Lib Dems won control of Amersham town council last week, with no Labour or Green councillors. To win here, they need a 14.6% swing. Remember they snatched Richmond Park from Zac Goldsmith in 2016, a tougher task with a 22% swing: Labour had a candidate but its activists turned out to canvass for the Lib Dems, a similar story in Brecon and Radnor in 2019. This could mean far more than just another cheeky one-off Lib Dem ambush. How each progressive party approaches this will reveal what hope there is for a remedy to Britain’s broken politics, and that rests mainly on Labour. The party needs to ditch its damaging rule obliging it to stand in every Westminster seat: standing aside would sometimes be the better part of valour. It would show Labour in a good light, displaying cooperation and a lack of hubris, recognising it alone is not the sole vessel for progressive values. Labour’s nastiest aspects are its closed, cultish, faction-ridden tribalisms, unappealing to outsiders – and many insiders too. Just on one occasion Labour did break that stand-everywhere rule, when it put up no candidate in Tatton in 1997 against the cash-for-questions scoundrel, the Tory MP Neil Hamilton, to give white-suited Martin Bell, an independent, a free run. Labour should do that again here. The Batley and Spen byelection coinciding makes it easy for a Lib Dem quid pro quo, standing down in exchange. Last week, a Tory was the sole rightwing candidate in 85% of council seats, where the progressive vote split two or three way, finds the Electoral Reform Society. But this is not just about weaselly how-to-win electoral mathematics, but understanding the stale old silos have to be broken open to blow in fresh ideas. Labour has been the elephant lying across the tracks, preventing change, keeping politics fossilised into the two monoliths, a cabal to keep out newcomers. The Lib Dems, Greens, SNP and Plaid Cyrmu wrote to the Speaker of the House “to challenge the prime minister’s persistent failure to give accurate information to the House of Commons”, listing untruths Johnson refused to correct. Labour would not join them, saying haughtily it didn’t follow other parties’ initiatives. That’s what has to stop. Instead, Labour must show an open mind in the face of the avalanche of motions being sent to its conference this year from local parties calling for adoption of proportional representation (PR). Labour For a New Democracy has organised a massive 214 so far, backed both by Momentum and by Blairites: Blair was for it, until barred by party dinosaurs. This new surge suggests a profound shift in thinking: all too often Labour people hate other left-of-centre parties almost as much as they hate the opposite factions within their own party – and more than the real enemy on the opposite benches. From the top, messages have been mixed, with Keir Starmer indicating a tentative pro-PR openness. Some fear that backing it now looks like a losers’ charter, but not if Labour embraces it with genuine democratic intent – try some open primaries for all voters to help select candidates, for example. It’s time to strike deals on seats and to seal a bond to create a non-Tory coalition at the next election, pledged to voting reform. How parties behave in Chesham and Amersham matters.