you're here in the uk right?well whilst vegging,personally i wouldn't bring them in at all at night,i have 3 going outside at the moment and they stay outdoors at all times,and are vegging along nicely,in spite of all this rain,i'd save bringing them in for flowering

plus the fact plants growing outside aren't as sensitive to small amounts of light during their dark time,they have the moon,light polution from street lamps etc which are just hps lights,so i wonder if they spend their veg period outdoors,if they might just be more forgiving if you have any little light leaks when you do start flowering,who knows heh