I have the leak from the plastic coolant junction box on the side of the head as well, you wont see the fluid on the floor as it collects on top of the gear box, I find that the loss only happens during the colder times in the winter and mainly when the car is sat for awhile. At the moment I am just topping up with G12 as required, getting the part changed is around £100-150 with labour depending on if the palstic part needs changing or just the washer.