Hi,
We did the water pump on a TD5 disco a couple of weeks ago, it was using a lot of water as you mention with yours but when we first checked the car it never lost a drop( left it running on the ramp, roadtested, rad cap on and off etc) but what we found is that if the water level was down at all it wouldn't leak. With the coolant filled right up and the engine upto working temperature check on the n/s of the engine,the housing for power steering pump and water pump. Look up from under the car and should see the tell tale hole in the casting which is where the coolant leaks from if the water pump o ring seals fail. The tell tale hole is on the side of the housing about 5mm in diameter, not the easiest to find as you can only see it from underneath.
Unfortunately TD5 heaters aren't the best, as the modern engines become more efficient the less heat is produced as a biproduct of combustion and so the colder the heater is.Hopefully it will improve once your water leak is found and repaired and give you a bearable cab.
cheers,
Stevie.