I have just mixed the D76 powder to create the concentrate.
And I stored it in a transparent plastic bottle.
I wonder if that's harmful; whether it may deteriorate because it's in contact with light.
What you want is called an accordion bottle. These are opaque, plastic bottles shaped like a cylindrical bellows with a cap on one end.
To use it, you pour in the liquid to be stored, compress the bottle until the level rises to the top and then install the cap. This customizes the size of the bottle for exactly the amount of liquid in it and eliminates almost all of the space for air that could react with the contents.
Keep it in a dark place: the transparency or otherwise of the container then does not matter. I keep my mixed chemistry in transparent containers in a cardboard box.
However air is generally a more serious problem: the bottle wants to be nearly full and well sealed.
The best storage will be a brown of green glass filled to the top and capped with little or no air space above the liquid level. Glass is best because it is impervious to air and will not contribute contaminates to the stored solution. As you use the concentrate or stock solution, add clean marbles to maintain a minimum air to fluid junction.
Next best is squeezable plastic bottles designed for chemical storage. Next is most any plastic bottle.