Leaks in the cooling system
Avoid products which claim to block any leak from the inside (Holts Radweld and similar). They can clog more than the leak itself, the heater matrix or the smaller passages inside the engine might suffer. Do it properly and thoroughly instead.
First of all, locate the leak. Look for the usual whitish residue around hoses and clips. If a hose is ruptured, replace it (on the roadside, you can use Holts Hoseweld to bandage a hose but replace it with a new one as soon as you can). If the heater matrix is leaking, replace or repair it.
If you found a hole in the radiator, use Loctite Quick Solder Radiator Repair or similar. It looks very much like a child's dough but it is made of two components. When you knead the two layers into one, it becomes soft like a putty but will harden like brass in an hour. With this putty you can fill any hole or leaking part. As this is an external repair (and you wait for the glue to harden before you restart the engine anyway), it presents no risks for the cooling system.
Alternatively, you can go to a radiator specialist who can simply solder it the traditional way. Still significantly cheaper than a new radiator...
The fuel heater leaking on the thermostat O-ring seal seems to be a typical problem. The same part also appears on the CX 2.2 and 2.5 diesel, 2.5 turbo (both I and II) as well as some BX 17/19 D engines. It is mounted on the side of the engine head on the gearbox side, covering a large hole. It is in direct contact with the coolant. The leak results in fuel flowing back to the tank when the engine is not running, and bubbles getting into it when it runs. A pressure test will not reveal the failure because the thermostat will tend to be pushed out and this will squeeze the O-ring and make it seal temporarily.
A telltale sign is when you can start the engine on the first crank but it just dies after a few seconds, and won't restart until you prime it using the small pump on top of the fuel filter. There is no replacement O-ring available from Citroën and it is accessible only with great difficulty. You can buy the whole heater for an exorbitant price and it takes a lot of work to change it, too. You should try to locate an O-ring of the same size from another source.
Note that Lucas, CAV and Rotodiesel pumps are extremely sensitive to air in the fuel and will resist all attempts to get it working properly under such conditions. The effect will sometimes be subtle, sometimes brutal: the engine might have problems with idle speed (both lower and higher than usual) which seem to correct themselves suddenly. But then, you can also have it refuse to drop lower than 2000 rpm.