When should I harvest my potatoes?

I am just wondering when is the best time to harvest maincrop potatoes planted in April? Is there a certain amount of weeks needed? Or is it something related to the flowers on the stalks?

Some of them have had flowers in the last week or so.

Also, instead of harvesting them altogther, is there a certain amount of time that you can leave them in the ground after starting harvest or should they be dug up altogther?

Thanks,

John........ a first timer..