Potato Q

If a tattie bag from last year has magically sprouted a tattie plant this year....will it actually grow potatoes?! Will they be safe to harvest and eat?
And how do I know when they are ready?

I have no idea what variety they are, they were from the "grow your own"....either Rocket or some other baby new type

 

Fab! Tatties for dinner at some point!
How do I tell when it's harvest time?

 

All good advice from John Snowbaby and you can tell when they are ready to harvest either by the foliage going a yellow colour and wilting as it dies off,or you could have a gentle rummage amongst the soil to see if there are any tubers ready,but this is going to be quiet a few weeks away yet if they are only just growing inside the bag.Dont forget to keep them watered in dry weather but try not to get the bag too waterlogged