Refrigerant leak in Xsara Picasso

For the past few months, my car has been losing coolant from the thermostat housing. I've replaced both the housing and the thermostat, but it still leaks when the car reaches normal operating temperature. After the last housing replacement (I've replaced it twice, once from an auto parts store and once from Citroen), it no longer leaks from the base, but now it's leaking from the drain plug. It's tightened properly, but it seems that due to pressure or temperature, the coolant is leaking out wherever it can. I'm hoping someone can offer some guidance; this has become a matter of pride, and I need to figure out what's wrong.
Thanks for your help.

Tell me one thing: does your vehicle overheat, or does it function normally, or does the temperature gauge indicate that the car is above 90 degrees?

First, I apologize for not replying sooner; I haven't had internet access over the weekend.
The car is running at a normal temperature, but naturally, when the dashboard indicates that I need to top up the coolant, I do so. It's clear it's low on water because the reservoir empties, and there's a characteristic burnt smell from the leaking water.
One question: how can I tell if the thermostat is opening or not? I understand that heating it with a lighter should open it.
Thank you for your interest.

A couple more details. Does the cooling fan turn on and run for the usual amount of time?
Are the hoses very hard when it's hot?
Are the upper and lower radiator hoses the same temperature when the engine is at operating temperature? Do
the spark plugs have grayish deposits?

I haven't seen the electric fan (I assume you mean the cooling fan) working for a while. It's also true that I don't let it overheat; under these conditions, I try to drive as little as possible. I'll try letting it run until it kicks in.
Of the two most accessible hoses, the 50mm one that runs from the thermostat housing to the radiator is almost cold and has no pressure. The other one, which runs from under this housing to the expansion tank, is very hot. I haven't checked the lower radiator hose yet; I'll let you know this afternoon.
As for the spark plugs, it's a diesel, and at least on the outside, I don't see any fouling.
To understand all this a bit better, could you explain which direction the coolant flows?
Thanks.