Growing spuds in containers, help!

Hi,
I am a complete novice and took it upon myself to try growing potatoes in containers on my patio.

So I leaped in with both feet bought a couple of large tubs and a few potato bags (40cm diameter, 70cm deep), grabbed a couple of bags of seed potatoes and did a quick google on what to do next. So far so good....

Read up on building up, where you plant initially in 12-15 cm of compost and as the plant grows you add more compost until the level is close to the top. Seemed simple.

The plants started to grow nicely, so a bit more google and I read about varieties...
Discovered that what I had bought were 1st and 2nd earlies, wilja & pentland javelin.

So back to google... Read that you shouldn't (or it's not worth) build up 1st's and 2nd earlies because you don't have time to do it before they are ready for harvesting.

Can anyone help by answering the following questions:

1. Is the building up process null and void for earlies? I.e is the above true?
2. If so, then what should I do?
3. Leave them as they are, accept a small crop and move on to next year?
4. Could I lift the plants (1st earlies are about 10cm tall, 2nd's are about 5cm tall) and fill the pots and bags and replant again nearer the top of the pots and bags?

I very much appreciate any help you can offer.
Best Regards
David

 

Many thanks John for your prompt and informative reply, much appreciated.

 

I "build up the compost" in my Potato containers (1st and 2nd early varieties). It is true that I don't get any/many spuds in the top layer, but as I see it:

If I plant them deep they have to use energy, and time, before they leaves get to the surface and can start converting sunlight into energy

I need a lot of compost to hold enough water for the plant

The roots don't go down very deep, so if I plant them at the top (of a deep bag) the water in the bottom compost layers won't be being put to as much use (although it will travel upwards by capillary action, IME the compost at the bottom is wetter, and that's where I want the plant's roots to be - they do drink an awful lot when they are close to maturity)