Not sure, but from my observation, I think that potatoes are a crop that is controlled by day length. They die back well before frost, unlike other plants in the tomato family. So my earliest potatoes are starting to think about dying back now, and my latest potatoes will be harvested in late August or early September. So I guess I am thinking that it is too late to plant potatoes for this year. They are one of my earliest crops to plant in the spring. I presprout them inside and then plant them out when I plant my peas or a little bit later. I think that the longer they have to grow, the larger the potatoes will get.

We buy fingerlings from the farm stands. I don't know how long those take to get to that size, but it is something to look into.

I am harvesting fingerlings now, a variety called Russian Banana. Nice flavor and a kind of waxy texture great for roasted potatoes and potato salad, but don't try to mash them - gooey. They are my earliest potato. I also am growing Yellow Finn, which is like Yukon Gold, but is a longer keeper than any other potato I have grown over the years, including Yukon Gold. YG produces a bit more per plant, however. My third variety this year is Butte, a baking potato that has a wonderful moist crumbly texture. I also have a few that I missed last year that resprouted, a kind that is supposed to be more bug resistant, King Harry. It does seem less bothered by bugs than other varieties, but is a rather ordinary white potato to my taste.

I now grow RB, Butte, and either YF or YG every year. They all keep well and have good flavor and texture. I've tried a bunch of other types, including Swedish Peanut fingerling, at least three of kinds of red-skinned (all short keepers), a blue-skinned called Caribe, Katahdin (white), one other type of baker, but none had the combination of good flavor & texture and long storage that I want in a potato.

I presprout my potatoes in spring (just set them in a warm bright room on a surface that will hold them still; I use an egg carton.) Then I plant them outside with the peas in April. I harvest as the vines die. In between I squash a lot of potato bugs and their eggs, but I really like the flavor of fresh potatoes right out of the garden as well as having them until March most years, stored in the cold cellar.

Below is a link to where I get my seed potatoes, family farm in way northern ME. They have a wide selection of varieties and good guides to growing potatoes and how to choose a potato for different uses.

Here is a link that might be useful: Wood Prairie Farm

The weather is different in great britain, but I found an article about growing winter potatoes - planted in July or August - which I'll link below.

Here is a link that might be useful: late potatoes at Telegraph