Scheduled time. Unlike a structured MA, you'll be much on your own, so imposing a timetabled structure and agreeing targeted deadlines with a supervisor is essential.

You didn't say whether its an OU PhD or with somewhere else? Doing a P/t one in an institution geared to full time can mean you miss out though if you make contacts with other PhD students regularly then that can help (though the different time scales may mean less in common as time goes on)

My PhD "stalled" 18 months ago, after several months of going nowhere. My end date came and the University contacted me to see what I was doing! I've not quit, even though I've thought about it. I was surprised that the department now want to help

I do know once I had passion, and maybe if that can be rekindled, I'll complete. I used to love research, and when I talk to someone about their (related) subject get all enthused with suggestions, its just when I retreat to the darkened room it evaporates!

As has been said, a PhD isn't the be all and end all, and really means little outside of academia. The opinions of many PhD recipients is by the end they had lost most of their initial enthusiasm.

I'm hoping to continue part time and get to my PhD transfer stage. I know if I do, then I'll finish. The lucky people have a good supervisor or a good department, and are supported/guided over hurdles - the rest of us need forums like these!

Supervisors vary greatly and I have known at least one student whose supervisor insisted his name went first even though he mainly glanced over finalised papers, but obviously publishing without consultation is a bit rich.

Some things to consider:
Validation by publication: If you've had data published it helps towards your PhD as saying your work has been verified by peers in the research community. Maybe you should be pushing your supervisor to be able to write a section on his next paper and in so get more experience writing and a longer list of publications.
Up by the bootstraps: My original supervisor had the idea that having known names on a paper increases its likelihood of publication, until you get your own known. You are able to then learn from the publication writing process. In my first year I wrote several papers onm populist subjects, that were tenuously linked to my PhD but did at least give me a grounding on paper writing if not anything to my PhD