Dennis,
Based on different Jaguar forums, I got the impression that probably the leak is from the water rail gasket. However, I looked there and could not confirm this. But as you suggested, I have to check it when the engine is running.
A positive thing is that I managed to replace the engine belts. However, I noticed that I had broken plastic heads of the EAC 2630 thermal valve on the thermostat housing. Two vacuum lines are attached to these heads. However, I am not sure whether these vacuum lines are really necessary. 
[[v12-engine] EAC 2630 Thermal Valve]