Politics will always make for rich drama. And contemporary politics is a particularly fruitful area. If Only explores the run-up to the coalition and its aftermath but falls flat as David Edgar fails to develop his characters in any way and doesn't say anything original about the nature of politics.

It doesn't have the humour of The Thick of It, which explores the same area, nor its insight. It's a sign of the Chichester audience that the loudest laugh is reserved for a joke about apostrophes.

Edgar's play is stuffed full of implausibilities. Like the worst of sitcoms, he has created an artificial situation for the characters - Peter, the Tory MP; Sam, the Labour policy wonk and Jo, the LibDem advisor - one that forces them to spend three days in each other's company (seemingly with limitless funds for transport but not enough to pay for a change of clothes).

The biggest implausibilities of all are the political ones: would the dissolution of parliament, a month or so before a general election, be a major political event? Would a party dump its leader a few months before an election? Neither seems likely to me.

Eve Ponsonby (Hannah)
Eve Ponsonby (Hannah)
© Catherine Ashmore
But that typifies the play: it's a giant mix-up of ideas, some lightly-sketched, some more coherent. This is emphasised most by the portrayal of the A Level student, Hannah, they pick up en route. Edgar's nod to the younger generation is the insertion of liberal quantities of 'like' into her speech. It's like it's the only way that, like, young people speak. It's a tough slog for Eve Ponsonby to try to make something from a one-dimensional character.

Edgar's play is a two-hour plea for common liberal values, where one-nation Tories seek common ground with the opposition. What Edgar misses is that it's precisely this common background and careers of politicians from the three major parties that are helping to fuel the UKIP surge. The "arms race to the gutter", as Peter puts it, would have been a much more fruitful area to explore – and it's one that Edgar has already covered in Destiny.

The cast do well to give a human shape to Edgar's caricatures, especially Jamie Glover's Tory MP, who shrugs off the trappings of privilege to reveal the moral heart of the play. But there's nice work too from Charlotte Lucas' Jo veering between the two parties and Martin Hutson's Sam, trying to keep hold of his principles. Director Angus Jackson stops it from flagging and there are occasional flashes of insight – but I see If Only as an appropriately-titled missed opportunity.