I am guessing there are a number of factors here that may have caused this situation.

The main one is the depth and diameter of the containers for two tubers (plants)

Add to that the poor light levels we have had this year, I think many of us are having lower than normal yields this year because of this, so that takes care of one possibility.

There is a limit to how much growth a plant can sustain in containers or in the ground even.

I would say each tuber wants around one cubic foot of growing space.

If there is much more the plant will not necessarily fill the additional space, and if there is less then the crop will yield a certain weight which can manifest itself as lots of little tubers or a few large tubers.

Variety can also play its part in this situation.

Similarly the 'food' content of the compost, potatoes are greedy feeders.

So I would accept that your yield has been a bit rubbish but I don't think it is necessarily down to the methodology you have used as many people get good results from this technique.

I would be interested to know your container sizes and what you filled them with, and what aftercare you gave them.

Thanks, Teegee for your reply. The volume of each sack was approximately 2.5 cubic feet.
I put a little manure in the bottom, then 4-5 ins of my compost, into which I set 2 potatoes which I'd bought from the local garden centre.
I filled it up gradually with a mixture of garden soil and multipurpose compost, watering it as I thought necessary, adding blood,fish and bone now and again.
I guess this was incorrect ?

It sounds like your bags were a bit too deep.

Perhaps as you added more layers of soil the lower layers become too compacted and possibly waterlogged, thus affecting tuber growth.

When you consider that when growing them in a bed in the garden and then you earth them up,(which is effectively the same as adding the soil to the bag) the tubers are only covered with a couple of inches of soil.

If I was growing in bags I don't think I would top up more than say six inches above the seed potatoes.

Then when you consider the low light levels I mentioned this won't have helped.

So apart from the depth I don't think you have done anything wrong.

Incidentally what variety were they?