This has been a fascinating debate: I didn't really expect to have so much feedback and so many suggestions. Thanks to everyone.
As part of my work, I write computer code and I don't find it a problem to do this on time. I also write 3,000 word editorials for a national PC magazine and have never missed a deadline.The common thread in my professional life is that these deadlines are imposed from outside. Perhaps I'm not treating my fiction writing in such a professional way.
I think, given the size of the average novel, splitting it into chunks and treating each of these as a mini-project sounds like a good way to go about it.
As for planning, there seems to be no agreement in the advice I've received either from books or from individual writers. The way that feels right to me is to have an idea of the main milestones in the story, to create a good cast of characters and then to propell them (or have them propell themselves) through the plot. I don't like the idea of planning too thoroughly as I think that robs the writing process of the joy of surprise. I appreciate that other people see this completely differently and that different genres probably require different approaches.
Anyway, fascinating stuff. I really want to feel that when I start again, I am going to finish and I think the keys to that are a roadmap of where I'm going, a cast of interesting characters and the discipline to sit down and get it done!!
I have a friend who, when she was writing her novel, gave me £100 in £20 notes. The idea was that she emailed me 4000 words a week (she wasn't working at the time, but i still thought that was a lot of words!) and for every week that she stuck to the deadline, I posted her £20 and for every week that she didn't I posted £20 to charity.
Some people really need deadlines. If that's how you work, don't beat yourself up over it - use it!
Also be aware of how hard novel-writing is. I agree with what others have said - once you know you can do it, it makes it a lot easier to just plough through and finish.