It seems as though theatre has been waiting for a show based entirely around mental health.

We live in an age of anxiety, as we are often told, our nerve endings frazzled by instant communication and rolling news feeds. And who better to take on that challenge than Calixto Bieito, the Catalan director renowned for his daring approach to theatre and who himself suffered from an anxiety disorder as a child?

Add in the impossibly dexterous Heath Quartet, four accomplished actors and a script drawing directly from seminal philosophers Soren Kierkergaard and Robert Burton (among others) and all the ingredients are in place for an enlightening exploration of our inner lives.

The String Quartet's Guide To Sex And Anxiety doesn't quite play ball, though. Sure, there are moving monologues about various forms of mental trauma, from overwhelming grief to crippling everyday anxiety.

A tear-stained Cathy Tyson tells a powerfully detailed story about the death of her character's young son, while Nick Harris treads the line between humour and horror when disclosing his character's endless quest to quell his nervous condition. Mairead McKinley's evocative role, meanwhile, shows how overthinking the sex act can lead to an unhealthy fixation.

In between, the Heath Quartet play twitchy, sometimes rousing renditions of Beethoven and Ligeti pieces. While these masterful numbers fit the general tone, the uneasy mingling of soliloquoy and song doesn't make for a cohesive overall narrative. Too often the characters' spoken interjections resemble individual sketches with no relation to each other.

The show becomes fragmented and shapeless in its second half when the monologues drop away, leaving the quartet largely to its own devices. The String Quartet's Guide To Sex and Anxiety is an odd proposition, intriguing in its subject matter and reluctance to conform to theatrical conventions.

But then Bieito was never going to hand us anything on a plate.