As the foliage starts to brown and die down.

Your a bit early as yet, If you want to know when they are ready, use your fingers to dig around the base of the plant and if you find potatoes, you have potatoes!

Earlies will be much smaller than main crop but should still make the size of a tennis ball!

Depends if they are early or maincrop. I have started to harvest my 2nd earlies (Winston) this week, just taking about 6 at a time over the past couple of days, i.e. enough for that evening's meal. My plants haven't flowered, but then they didn't last year before harvesting. Rootle around gently and if they seem an OK size (assuming earlies) then you can harvest. I also have Belle De Fontaney and these are still pretty small so I have left those alone.