Applying for a PhD - how and when to broach the fact that I have previously left a PhD program I am applying for a funded PhD project for which I am quite well suited in terms of academic record and experience. A few years ago I began a PhD program which in hindsight was not a good fit and I decided to leave that program at quite an advanced stage. I am personally secure with this decision and think that in some ways the experience even strengthens my application, and I want to be open about it. However, I am not sure when or how is the best way to bring it up in the positive light in which I see it.

On the one hand I could place my enrolment on my CV under PhD Student or similar and discuss some of the advantages in my cover letter, but neither of these seems an appropriate place to discuss my reasons for leaving and I don't want to raise a red flag without having the opportunity explain.

The other option is to not mention it until the interview stage. This I could easily do as I was also employed throughout the time that I was enrolled and so would have no employment gap, but this feels dishonest and also denies my the opportunity to capitalise on some of my skills in my cover letter. 

My preferred solution is to put it on my CV and discuss it in terms of skills and experience in my cover letter but not to broach the question of why I left until it comes to interview. My hope is that in the right light it will not in itself prevent me from getting to the interview stage. I am looking for a bit of feedback on that plan, or some alternatives if it sounds career-suicidal. I am particularly keen to hear from academics or admissions tutors as to how you would look upon such an  application.
For a variety of reasons you should reveal it early as part of your education in the CV or other materials. But you don't need to give any reason there. Just that you were in a doctoral program (and maybe ABD) from such a date to some later date. 

You are probably required to give transcripts of all educational endeavors, and it would be a lapse to omit any. 

There are a lot of reasons for leaving a doctoral program and only of a few of them reflect badly on the student. Not passing comprehensives might be a negative and being expelled for ethical lapses would certainly be. But those are only a few of the possibilities, including just becoming disillusioned with the program or with an advisor. Money. Family. Medical. 

But later, in an interview, you will probably be asked and then is the time, IMO, to give reasons. 

And, if you think the experience strengthens your case, then, in a Statement of Purpose you could say how it supports your future goals.