how do i fix my water pump on my rover 45 1.8 petrol?

my rover 45 1.8 petrol is overheating. it is blowing back out of filler bottle. i have looked for leaks under the car but none found. when running there is no water getting pumped back in to water filler tank. could it be the pump at fault. or thermastat fault. or headgaskit. no water in oil. i just carnt figger it out. can anyone help. many thanks