I went to a book launch overlooking Westminster last week, and found myself surrounded by those uncategorisable figures who have inhabited the political world on the left for some decades – the likes of Jonathon Porritt, Peter Hain, Andrew George, Anthony Barnett and Caroline Lucas.

What had drawn them all was the opportunity to launch a book of essays urging cross-party cooperation on the left for the next election, to avoid what looks otherwise like a get-out-of-jail-free card for the Conservatives.

It is called The Alternative, and I’m rather proud that my own essay, on a new approach to economic policy, provides the second chapter (the first is by the most optimistic commentator in western Europe, Neal Lawson). Whatever people say about those chapters, the book as a whole will provide a background text to the Liberal Democrat conference in Brighton this weekend, after another major setback (the Brexit vote); and we will hear a great deal more about building a cross-party platform on the left.

Lib Dems are all optimists at heart. They have to be. But there are some rather obvious barriers to a pan-left platform at the next election: three in particular.

First, there are no mechanisms in any of the formal party structures to organise a formal pact. It will take every particle of Paddy Ashdown’s considerable energy, through his new More United platform, to make it remotely possible for anyone to stand down in favour of anyone else.

Paddy Ashdown (centre) campaigns for remain before the EU referendum, with Neil Kinnock and David Cameron. ‘It will take every particle of Ashdown’s considerable energy, through his More United platform, to make anyone stand down in favour of anyone else.’ Photograph: Stefan Rousseau/AFP/Getty Images

I also admit to a bias here. I wasted my whole 20s fruitlessly helping to negotiate between Liberal and SDP ambitions in Oxford, and I’m not keen on doing the same with my 60s.

Second, Labour party members dislike each other so much that they seem unable to work together, let alone with anyone outside. And angry purists on the left seem to hate people who think slightly differently more than they hate the other side.

Third, and rather less obviously, the formal parties of the left – Labour, Lib Dems and Greens – have long since become bored by economics, forgetting that any winning platform must include a challenging recipe for creating prosperity.

There never was a period in recent history when a better way of doing this was needed more, given that we are subservient to an economy devoted to funnelling money upwards to billionaires, from where it patently fails to trickle down.

Angry purists on the left seem to hate people who think slightly differently more than they hate the other side

Yet the left has abandoned this agenda to the right, concentrating instead – and rather patronisingly – on welfare. Another matter entirely.

We might address one or possibly two of these barriers before the next general election, which we have to assume will take place in 2020, but the chances of tackling all three seem pretty remote.

But don’t let’s give up. There are two things we must do to bridge the chasms that divide the left. We need to so some cross-party thinking, as set out in my new book, The Death of Liberal Democracy? – written with Joe Zammit-Lucia and also published in time for conference season. We can also use our political institutions to make things happen.

That sounds obvious, but it isn’t. All too often the opportunities to reach out across party lines, to cooperate to change the law in parliament and beyond, get stymied because of obscure and irritating rivalries between opposition parties – and because their priorities and strategies are different.

Yet the government has only a slim majority. Imagine if we could agree across party lines to use parliament for shared objectives.

We might not be able to have an impact on the big, symbolic policy areas – shifting the NHS or changing fiscal priorities – but we could do something.

If we wanted to break up Royal Bank of Scotland into local lending institutions, we could hammer out a cross-party strategy to achieve that. If we wanted all public service contracts to be in the public domain, ending commercial confidentiality for public services, we could do that too.

It might sound unexciting. It might stretch parliamentary conventions. There will be squabbles about who can claim the credit. But if we can achieve things together – then and only then can we be united enough to be a bit more ambitious.