It might depend on how much compost you placed in the bag before planting the tubers. I would carefully remove some of the compost from the top of the bag with your fingers as the tubers might be sitting higher up in the bag. Even though the flowers have finished, (sometimes lack of water can make a plant come into flower early) it could still be a little too soon to harvest second earlies.

Thanks . You may be right about the level of the base compost, though I thought the tubers worked their way down but I will be patient and wait. BTW what happens with irregular watering?