semi broody girl, how do I make her go totally broody

[thesockmonsterofdoom]

I want to hatch some eggs, I cant afford asn incubator.
One of my girls has been semi broody for ages, she sits on all the other hens eggs, keeps sitting when they are removed. Problem is she then gets up and behaves like a normal chook the minute I open the door for free range time.
I have cleared out the eglu so she can have that to hersef, should I give her a couple of non fertile eggs to sit on to make sure she is serious before I get some eggs.
Then when I do get eggs will she sit for long enough, ie do they only sit for 21 days? Sure I will come up with more questions in a minute.
I am responsible and I have thought about cockrels etc.

You can give her some eggs to sit on but this won't tell you if she'll sit the entire time-it seems to me like you'd need an incubator for back-up in any case. Perhaps you could borrow one or make your own using things you already have-i.e. an old lamp.