Hallo, it's been interesting reading this thread - so many resonances with life chez nous. My DP's depression is compounded by an alcohol dependency, and in both, he's 'high-functioning'. Most people wouldn't know he was depressed, nor that he drinks, as he confines it to the evenings.
Rorogeorge, as cestlavie said, you need to give him an ultimatum. My DP's depression developed over 3 years until his behaviour drove me to getting the house valued (he'd often talked of splitting up and selling up, but never did anything about it). I showed him the estate agent's brochure, left out leaflets for flats that I was considering, wrote down a 'spider diagram' of his behaviour and how it was affecting me and my daughter, and what alternatives we all had. and told him that either he and I saw the GP and organised therapy, or that was that. The ultimatum worked, we are now seeing a very good counsellor and the improvement in him over the past couple of months is marked. Still a long way to go, but he's more affectionate and responsive, and where before, he said everything seemed dark, with no hope, he now sees that there might be a way forward. He won't take ADs but, so far, the therapy is helping.
The trouble with depression is that it's an invisible illness and family/friends aren't always able to understand - or cope with it long-term. But like any illness/disability, we have a choice whether to go, or stay. In my case, I'm staying, but only because DP made the choice (after my ultimatum) to seek treatment, and only because he's been able to tell me that, ultimately, he knows that he's happier with me, than without.
You know, what we need is an organisation like AlAnon - DepressivesAnon would be good - somewhere, like this thread, where the families of depressives could go, for support and sharing.