"With earlies, wait until the flowers open or the buds drop; the tubers are ready to harvest when they are the size of hens' eggs." quote from royal horticultural society website..
https://www.rhs.org.uk/advice/grow-your-own/vegetables/potatoes
My first earlies have flowered, I waited a bit longer until the flowers had finished as the first test potatoes i dug up where a bit small.
Looking good.
Looks like we are going to have a good yield, this year. There's still about 4 or 5 times more to come out of the ground :-)