 4.Any gender and aged 18 years or over 5.Meet criteria for treatment resistant depression Fekadu et al., 2009a; Cleare et al., 2015: current episode has not responded to at least two antidepressants given for at least 6 weeks at minimum therapeutic dose defined as fluoxetine 20mgday, paroxetine 20mgday, sertraline 50mgday, citalopram 20mgday, escitalopram 10mgday, venlafaxine 75mgday, duloxetine 60 mgday, mirtazapine 30mgday, tricyclic antidepressant 125mgday, and dosage as guided by the national Maudsley Prescribing Guidelines or BNF for any other antidepressant. Please note, relapse whilst on an antidepressant also counts as a failed treatment trial 6.Current antidepressant treatment has remained unchanged for 6 weeks 7.Provision of written, informed consent. 