After posting here and getting great support for a slightly red-herringish alcohol problem, my husband and I attended a joint counselling session with an excellent private psychologist.
(Thanks Atilla the Meerkat for some really enlightening comments about abusive behaviours being played out. Very insightful!)
The psychologist was brilliant. She really laid it on the line and it was a hallelujah moment for me! Being heard and understood was so freeing for me after five years of trying to get through to my husband.
She told DH that he needs to "go deep" and commit to a course of psychotherapy over a long period, up to two years. She told him that he was a victim of childhood abuse and neglect and that the trauma of those early experiences has affected his personality. She told him (and he HEARD her) that our marriage is not going to survive if things don't change.
I had kind of bundled up his issues as "alcholism/drinking problem", then "anxiety" or maybe "anxiety and depression" but she was able to see through the anxiety symptoms and realise that actually he probably has a personality disorder brought about by his incredibly controlling father and a childhood of being bullied, repressed and emotionally manipulated. Essentially, his parents' lack of emotional intelligence stunted his emotional development at the age of about eight.
She also said that while he is not abusive towards me, the pattern of behaviour he observed between his parents is highly abusive and that he is subconsciously re-creating those patterns in our family. I don't respond in the way he expects (the way that his mother responded to his father) and that creates lots of frustration and anger in him. I'm so glad my boundaries are still firm after five years and that there is some hope he can learn to recognise and change his patterns of behaviour.
I'm under no illusions that this will be an easy process for either of us but I'm feeling hopeful as we take the first step together. Considering that this time last week I was making appointments with divorce lawyers, it really feels like a break through!