>I got a bit closer last night. I found an example last night which actually showed the entire code in one file rather than in little bits which don't explain how they fit together properly. So I used this example and got that to work. I then adjusted my Master page to use very similar code and now the mask is applied when the textbox has focus, I can enter only numbers in there, but when I leave the textbox, what I've entered disappears!
>

I just realised what is apparently going on:

If the number entered is shorter than the mask, then it gets blanked out. If it fills the mask fully, then it is visible. Is this how this is supposed to work?