Thanks all for the suggestions

Have not been able to find the issue as yet after a few attempts.
We don't use any village/mains nor have any outside connections, our supply is a 60m deep 4" well with a "smurf" water pump. We also don;t have any water meters nor one way valves. I remember a one way valve discussion years ago, but was told no way the water will flow back down into the well.
Plus, the smurf water pump is very powerful I doubt the house water pump is any more powerful nor would have backwards water flow into the well. I think the water pump would also have some mechanism for one way flow, and would not let water flow backwards into the well (I could be wrong here).
We don't have any water meters nor can I see any easy way to check if there is any back-flow into the well, although I seriously doubt that would be happening (a clear PVC pipe would come in handy here but I don't think they make those).
The reason I make this statement is when we are losing water in the main tanks, the smurf well pump is pumping constantly, filling the tanks, but the wtaer loss is fatser than the smurf can refill the tanks (which is also another quite strange fact for me!!)
The problem is definitely in the garden pipes circuit - if I close off the main pipe (2") leading to the house with the ball valve, and close off the garden main pipe (1.5") with its ball valve, there is no water loss. As soon as I open the garden supply ball valve (house is still turned off) the water pump starts pumping and the tanks will empty in a few hours, but there is no obvious sign which garden tap or connection is leaking. The smurf well pump is pumping during this time trying to fill the tanks).
My next step is to cut the garden PVC in several sections and attach ball valves along it, then sequentially turn them off and see what happens.
I'm very confident it's not the house circuit, as when both the house and garden circuits are off, the water pump is quiet and doesn't pump. If the house circuit is turned on and the garden circuit is still off, the pump does not pump unless a tap is turned on in the house, then once the tap/toilet is turned off, the pump stops pumping, as it should be.