Do I harvest my Potatoes soon?

Our potatoes have been growing in a potato sack since mid-April. The plants themselves are getting really big (we've even had to tie some to a stake to hold the plants up). When do you reckon would be a good time to start harvesting? Not sure what variety they are so dont know if they are supposed to flower.

Definitely wait until they have flowered.  Dig around a bit until you find a few potatoes and see what size they are.  You can do this without removing them from the plant. 

There is nothing more disappointing than tipping out a plant to find a small collection of marbles. Leave them longer rather than sooner, they will still taste incomparable.

Not all potatoes flower, so  that's not always a reliable guide. I agree, you could scrape away a little of the compost to see if you can find any of a suitable size....it's even got a name with the old gardeners...it's called a broggle or broggling.

It's worth noting that it takes between 10 &12 weeks for earlies to grow from planting to harvesting.