Hi JayJay,
When the flowers appear on your Potato plants it is usually indicative, though this also depends on the variety, that they are ready to harvest and not that the tubers are beginning to form. Keep the patch watered throughout the growing season and when you see the flowers have a gentle root around in the soil to see what size your potatoes are. If they are as big as what you want them to be harvest as you need them and leave the rest to grow on until you need to or have to, blight or slug damage, harvest them.
Also can I suggest Potato/Tomato Planter Bags instead of 30cm pots. The Planter Bags are no more expensive than pots, are roughly 35cm in diameter and stand about 45cm high. You plant 3 tubers in each, so for a tiny bit of extra space you get 3 times the crop you might expect from one 30cm pot.
A really cheap and clever alternative is to buy the 100 litre bags of compost. Cut these in half, cut a few holes in the base of each, and you plant in these.
Don.