I don't want to put anyone off, but yields are always pathetically small growing potatoes in bags. At the very least they need feeding and watering very heavily to get anything more than a handful of small tubers.
Late season potatoes have a much longer growing period and tend to be larger. If you can get hold of some free or cheap timber from a skip or the reclamation centre to make a box around 3ft square and 4ft deep, and fill it with a good rich compost, you would produce far better results growing in this, and it wouldn't take up any more space. You can put the tubers in covered with a foot of compost and then slowly earth the plants up as they grow..

I agree with Bertie...