It seems to be a UK standard that you write the numbers from one to ten as words, but anything from 11 upwards should be written as figures. I’ve had drummed into me in several workplaces, and I’ve forced it on others; it’s also in the Economist and Times style guides. (The Guardian says numbers begin at 10… typically awkward.) But Jakob Nielsen makes a good case for a different approach, based on semantics and skim-reading.
In essence, he favours using figures in any situation where you’re referring to an exact number, including single figures; but to say ‘hundreds’ or ‘thousands’ as words, because you’re using those words as vague descriptions of a quantity, rather than a precise number. It’s hard to argue with the logic in this; but I wonder if I’m too ingrained in the old way to change now?