In my flat metro water is supplied two hours only. I need to install a loft tank to store water. Please arrange connection diagram.

Easy fix! Install the holding tank and connect the house system to a float switch on the tank.
This takes care of filling the tank.
You will also need to cut in a tee from the float valve pie to the bottom of the holding tank with a shut off valve.
This valve's impotent because it's normally shut while the outside water's on. However, once the outside water's been shut off you open the valve allowing the tank to supply your house.
The only down side that I can see is that you must shut the valve off before the outside water comes on. If you fail to close the valve and the outside water comes on it will fill and overflow the tank. Perhaps a electronic valve and a shut off sensor would take care of that problem. Good luck, Tom