When do potato tubers usually start forming?

I actually put mine in during the last week of March. And even though I had absolutely no intention of doing so, I'm going in for a fertle myself tomorrow; what with all this potato chit chat! 

I was lead to beleive when the flowers stop coming and die off it's time to dig them up. 

This depends on how big you like your spuds. I prefer my earlies as salad spuds so dig them earlier. In fact I am digging my Home Guard now and they are just right.