Lemon seedling

Hi All.
My wife has grown a lemon seedling from seed, its nearly 2 inches tall and looks very healthy.
It has been growing in the poly tunnel but the nights and early mornings are getting cold now here in Brittany. Where would the best place be for it to over winter and it what conditions ? .
your advice would be very much appreciated 

thanks , john

I've grown them in the past but not got any now, I'd move it to a bright cool window ledge, keep it just moist & keep an eye out for bugs, they're quite susceptible to greenfly & scale insects