Water coolant level constently running low?

I have a ford c max zetec and I'm constently loosing the coolant. I brought car to the garage and they found out that the thermostat was missing! (I only bought the car 2nd hand of a private person) so I got the thermostat replaced, although the car does not seem to overheat, I'm constently loosing water after driving just a few miles having to refill the water all the time. There seems to be no water going into the oil, no white smoke out of the exhaust and no squicky noises. The car runs very smooth still. The fella who sold me the car took it back to have a look at it (I found out he's a mechanic himself but didn't tell me) he said they found a leak and fixed it so I got the car back. But, although I seem able to drive a few extra miles now, it is still loosing water till the tank is completely empty. Sometimes the water starts to bubble, there's no excess pressure when I open the water tank lid and after driving and parking up the car you can see the water gashing out underneath the car at times. Are this signs of a broken gasket? What else could it be? I'm not very clued up with cars but want to make sure i'm not being fooled by the person who sold me the car either.

Pressure test. Plus, Ford has had some issues with the plastic reservoir bottle leaking at the seam. It only happens when hot and the coolant evaporates so it's hard to diagnose.

Well regarding the water you see coming out from under the car at times, this can be from the Air Con and nothing to do with the cooling system. As 'Q' says, a simple pressure test would show up any leaks from hoses, radiator etc. It is quick, simple and cheap to do. If that does not show any leaks then have a 'Combustion Gases' test done. A head Gasket leak between the combustion chamber and a waterway will allow combustion gases to enter the cooling system which raises the water level and pushes some out from under the header tank cap. The test is, again a very simple one, but this time is chemically based and the colour of the fluid in the clear tube of the tester changes if there's combustion gages in the coolant water.

Needs pressure tested.
That's where I'd use a special tool to pump pressure into the cooling system,
I usually take it up to around 15 psi and believe me any leaks really show up at that pressure.
Good news for you is you've seen coolant coming out from under it so it's most likely a hose or similar.
Not the head gasket!