Well that was relatively successful.  I now have >4000 words in my draft thesis, admiittedly mostly copied from other documents, but it's a start.  My technique was to use Scrivener with Pomodroido on my phone to keep me focussed.  Not entirely pain-free, but much, much better than procrastination!

The key thing, I think, is to try to write a bit every day.  If I can keep that up I think I'll make good progress.