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.