Make up a "header bottle" to fit into the coolant expansion tank, something like this.
I used a short length of top hose to be a push fit into the tank neck, which also blocks off the overflow hole.
Bottle was 1L, bottom cut off.
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header bottle, corrected.jpg
Drain and refill with coolant.
Keep the header bottle at least half full.
Open the system's bleed screws in turn to let out any immediate air.
Start up, and allow engine to reach full working temp, signalled by the cooling fans cutting in at least twice.
During warm up, use the bleed screws several times again.
Keep the bottle at least half full.
Some firm squeezes on the rad top hose will help expel more air, which will glug up into the bottle.
Raising engine revs to around 2000 for around 15 secs a few times will also help.
When no more bubbles appear, and no more air comes out of the bleed screws, job is done.
These diesels use so little fuel, it takes a while (30 mins+ on the 2.5) to achieve full temp.
When all done, leave the bottle in place, and let everything cool, probably overnight.
Syphon off any excess, remove temp bottle, and set coolant level to where it should be.
This is the recommended Citroen method for system bleeding, and it does work in forcing air out of the system.
Hope helpful.
C.