Christmas Potatoes

I am trying to grow potatoes in bags for Christmas for the first time.  I have turned compost bags inside out, put eight inches of compost in the bottom, three seed potatoes in each bag and covered them with about 4 inches of compost and some growmore (I think I am meant to add more as they grow through?) I have put a few small holes in the bottom so they dont get waterlogged.  When do they need to come into the unheated conservatory?  Is there anything else I need to do and should I expect the same number of potatoes as first earlies in a raised bed would produce?
Thanks Daisy

I've just ordered some from Dobies to put in the ground but i'm passing some onto my niece to grow in bags so we can compare notes