This might be a bit late, but as ours are just starting to flower I thought I'd add my limited knowledge!

It'll depend what variety of potatoes you're growing. If you're growing an "early" variety, then they come out as new potatoes, when the tops are flowering. If you're growing "second earlies" or a "maincrop" variety like Maris Piper or King Edward, then as someone else said, they'll come out as normal potatoes when the foliage starts to die back.

I think it's more usual to grow early varieties in planters (we're doing the same) but I'd double check what variety you've got in there. Earlies and second earlies don't keep very well, so they say you should only pull out as much as you're going to eat straight away. I also gather it's a bit of an inexact science so if you gently put your hand in and find a potato, and it's only the size of a pea, they need leaving a while longer!

We had the same trouble with the foliage collapsing, and that was due to too much water after some heavy rain. I earthed them up quite a bit more, and put in a few stakes (very gingerly trying not to damage any tubers!) and they've been ok since then.

Hope it all goes well!