I've sent myself back to re-read Tim's Writer's Resources and I think I really do need to print out and put on my desk this word of wisdom:
The enemy is not the badly written page; it is the empty page.
If there’s one rule you should write on a card and tape over your desk, this is it. A bad page does a lot of good things: it advances the story, it gives you a chance to work with your characters, it demands that you write all or part of a scene, it challenges you to describe your setting – on and on and on. (It even makes the stack of pages look a little thicker, which can give you a psychological lift.) So what if it does some of these things badly? You’ve learned one way not to handle that particular piece of material.
But the great advantage of a badly written page is that it can be rewritten. It can be improved. A blank page is zero. In fact, it’s worse than zero, because it represents territory you’re afraid, unwilling, or too lazy to explore. Avoid exploring this territory long enough, and you’ll abandon your book.
I've written 284 words today and now I have to go and get dinner. Maybe later I can make it up to 500. I wonder what a reasonable daily target would be.