They are supposed to be ready when they are in flower. Water as you would a normal pot plant, taking care not to drown them and a feed with a general purpose fertiliser diluted in a can of water once every 3-4 weeks should be fine. Make sure the soil is up to the top of the pot too as the tubers develop on the stems underground and the more soil around them the more the potatoes can develop.

I've grown them in dustbins for the last few years, just water them as you would any plant in a pot, don't let them dry out, feed them once a month with general fertiliser and keep rolling up the bag and earthing up to encourage more potatoes developing.Once the flowers are faded empty the bags and enjoy.

I have grown some like this for the past couple of years and used a piece of narrow drain pipe with holes drilled in it running the depth of the bag/container so that the water gets to the bottom without drowning the potatoes.

Thanks for advice everyone