A man after my own heart .. this year I have grown 6 rows of Charlottes ( approx 100 plants). Not to mention 10 x 30 ltr. tubs all Charlottes, started in the poly tunnel in February. Charlottes are second earlies by the way,
my favourite potato by far. My main crop are Sarpo Mira which are blight resistant, they will go into sacks and see me through the winter. If you want new xmas spuds ... start them now, as August may be too late depending on where you are geographicaly.
Ideally when you buy your seed potato, you should take note of whether it is a 1st or 2nd early... the maincrop is the last one to go in the ground.
The foliage will tell you everything you need to know. If it stands upright, then the plant is still growing. When it wilts and collapses, then you know it ready to harvest.
If you leave them in the ground for too long, you may find the slugs move in, it really is trial and error. Personally I leave them in the ground and dig Charlottes as I need them through out the summer.