Wow! Thankyou both of you, that's going to be a great help.

If my software ever 'gets off the ground', I hope it will be useful to other people on here, possibly.
The idea is that it searches websites for articles etc that contain 98% known words, as referenced from a vocabulary list (it will ignore proper nouns in the text, for now). The weakness at the moment is that you have to build that list yourself. The other weakness is my poor coding skills, but I'm getting there...

Maybe I can make some vocabulary 'tests' which assess what level they are at, and suggests a list for them to start off with? All for the future.