Probably a very silly question but is your system pressurizing?
The reason I ask is that on my Freelander 1.8 and my Rover 25 Diesel both suffered from rather dramatic coolant losses. I could never track it down or find any leaks anywhere. I checked clips, hoses, bolts, water pumps and nothing.
I was refilling every couple of days but I was never getting overheating or any issues that impaired the performance.
I replaced the expansion caps on both vehicles with genuine rover ones and the issue was literally fixed overnight and very cheap.
As I said a silly question and you'd probably already thought of it.