to get lots of potatoes fill the bottom on a large container only up to 4 inches or so, plant seeds and as the top plant(leaves, stem) grows build up soil around it till you get to the top then dig in! a compost with lots of nutrients is needed e.g. compost,soil from growing bags etc.

Make sure you site your bag/container somewhere that it will get full sun and keep well watered. You shouldn't need to use fertiliser if you are using bought in compost.

The problem we found was getting enough water in to them. They need a tremendous quantity of water to produce lots of tubers.

Yes, plenty of sun, plenty of water and treat yourself to a packet of potato fertiliser. As you are watering regularly then any nutrients in your compost will be washed away very quickly. I found Sutton's Foremost worked well for me in past years. When the growth reached the tops of the bags, I inserted four canes in the bags and tied string around them to give support. When you finally start to harvest the crop, it's like finding nuggets of buried treasure.

Tintin if you've got space t grow your potatoes in the ground you'll find it much easier.