Id like to know this one as im growing mine in sacks and the leaves and stems are huge...i was told you have to wait for them to flower but im not absolutely sure.

mine are the same in sacks and are massive. are yours lates or earlys

If they are main crop potatoes they need a minimum of 16 weeks from when you planted the tubers, can be even longer. Keep your sacks very well watered and after the 16 week mark gently work your hand down into the sack and feel around for potatoes if they are of a reasonable size then upend the sack and harvest your potatoes. You can try to just pull off individual tubers and leave the rest to keep growing.

thanks moon grow you ansaw a lot of my questions and is always great advice hope you dont mind.

Nope - though I am thinking of writing a blog on harvesting potatoes so I can point folk to :-) Purely to save me typing it many times.