This Broody Malarkey

Hello
I have only owned my 2 girls for a year now.  They started laying early on this year.  Since then they have spent the majority of the time broody.  I bought them eggs (which were duds  :x ) then got them day olds - which they brought up very well.  Then they laid a few eggs and have gone back to being broody again. :?  They don't seem obsessive about sitting but will whizz into the nest box as soon as my back is turned  :roll: .
I'm not letting them sit as I'd like a few eggs to eat but they seem to be persistant.
My questions are - Is this behaviour seasonal?  Will they stop being broody in winter? (probably stop laying then too  :? )
As they are laying less a year does that mean they'll lay for longer?
I'm not letting them sit but they are remaining broody, if they aren't being allowed to sit how long will it take for the broody pill to wear off!
Sorry for the inane questions but I'd like a few more eggs instead I've got growling, stroppy chooks!

Thats really helpful.  Another question are pure bred bantams more likely to go broody than cross breeds?