Thanks all for your suggestions. I should've perhaps given a bit more history...
My daughter has already been through CAMHS where as is often the case, she barely engaged. She is currently having a specialised therapy (DIalectical Behaviour Therapy) but attends sporadically and there is always so much else to deal with that the school attendance/lack of motivation is barely addressed . TBH her non-attendance started in year 10 as a result of friendship difficulties and eventually she moved schools. She has no such difficulties at her current school but, as I said, is depressed, although I have to say that doesn't stop her enjoying a lively social life...
So, quietly suggests, she is interested in hocus-pocus (inc. hypnotherapy) and I'm thinking that if she believes it may help, it may help, placebo all the way. The vitamins haven't had the desired effect - she's taking vitamin D and fish oils.
She's a bright girl with a block about attempting anything challenging so I thought a bit of 'suggestion' and relaxation could possibly begin to move her in abetter direction. I'm prepared to invest some money short-term as she'll be a long-term drain on the finances if she doesn't get enough GCSEs to do what she's thinking of for 6th form.
lettucesnow will definitely check credentials of potential hypnotherapists.
lateSeptember I hope there'll be some changes for you and your son. Its tough isn't it? And that's without all the teen behaviour stuff....