PeterT, No dispute about the technical need for a particular order being nought or zero. But it is the visible that interests me.
Turtle91, You cracked it. Capital and lower case mix, together with Sigil not being scripted to recognise single digit numbers. My particular thanks to you for thinking through something I should have spotted in the first place.
And thanks to everybody for several extra points, like the reminder and discussion on the use of <alt>.