2000 Land Rover Discovery 2 overheating

Good day

my vehicle has been running warm lately but not reaching the red zone. The other day on the highway it overheating with steam pouring out from under the hood. I opened the hood, and the upper radiator hose was disconnected thus there was coolant all over the engine, and thus an enormous amount of coolant gone from the reservoir. I got towed home, the next morning I really clamped down the hose, filled it with coolant, and drove around the block several times. The car started to overheat again, with now steam pouring out of the reservoir, and coolant really leaking out from beneath the reservoir, when I parked. I had put almost 2 gallons of coolant in the car, and a lot of it seems to be gone. I'm hoping this is a reservoir issue, and not a head gasket issue, or radiator. The reservoir cap screws on with a lot of difficulty, and what looks to be a overflow or relief valve drips coolant as I am filling the reservoir. Any ideas as to where I should start? Thanks very much