The recently-founded London Musical Theatre Orchestra is a massively welcome addition to the capital’s music and theatre scene. It enables book musicals to experience the richness of orchestration and power not heard other than in a rarity like the Glenn Close Sunset Boulevard at the Coliseum where a 42-piece orchestra could give symphonic substance to the Andrew Lloyd Webber score.

The lucky recipient, in this case also guest-conducting his own work, is American composer Jason Robert Brown whose The Last Five Years was recently performed without lustre at The St James’s Theatre but whose Honeymoon In Vegas delivers a completely different maturity and understanding of the musical form.

It also represents Brown’s move towards collaboration, slackening his book-music-lyric-directing grip on The Last Five Years, to the point of playing the pianist in its recent Anna Kendrick / Jeremy Jordan movie version, with a book by the movie’s director Andrew Bergman who also co-wrote Blazing Saddles with Mel Brooks. It’s also a progression away from pop-rock song styling towards something much more brassy and Big Band which better suits the story and the stage format.

The plot fuses a caper like Dirty Rotten Scoundrels with Indecent Proposal where mobster types hijack the love story of Jack and Betsy by offering her a weekend in Hawaii to pay off his gambling debt. Betsy escapes the hoods disguised as a showgirl, Jack has pursued her in a troupe of skydiving Elvis impersonators. Apart from Jack and Betsy, the characters are all straight out of Central Casting – it’s not Pinter, but it is funny.

Samantha Barks is so much better in this than in The Last Five Years, although for me there’s still a needlessly hard edge to her voice, but the big mystery is why Arthur Darvill is not more often seen as a West End leading man given how well he handles both the vocals and the comedy with an unforced tenor and a natural charm.

In support, Maxwell Caulfield felt a touch underweight as the James Caan character from the movie, but Simon Lipkin as a greaseball lounge singer and Rosemary Ashe as Jack’s snarky wheelchair-bound mother give tremendous value.

If this is a calling card ahead of a fully staged production, there could be a lot of support.