I have a 5000 litre s/s tank at ground level. A horizontal 22mm I/D pipe runs from base into house. What would be highest outlet I could expect and what flow rate?
The tank would only be refilled when empty.

The highest possible outlet would be equal to the water level in the tank. The number of gallons of water in the tank doesn't matter - only the "head" (i.e. height) of water above the ground.

Also - do you have any requirements for minimum water flow, in volume flow of liters/minute of flow speed of meters per second? Or is a mere dribble of water good enough for your purposes? The greater the rate of flow you want the higher the head of the water in the tank needs to be above the outlet.

"water seeks it's own level". Regardless of how much water you have in the tank, if you only have it 2ft. deep, it will only go up 2ft. In the pipe. You cannot push water up unless the tank is on a hill or with a pump.

Sorry, punched wrong key.