Thanks to all the input on this site I have just managed to replace the coolant tank on my 01 C4. The original tank had turned yellow and was cracked. The tank is also much bigger than the replacement tank. The old tank was a huge pain to get out, I had to drop the engine about 3 inches and eventually got it out. The new tank being much smaller went in really easy. I hope the smaller tank doesn't cause any problems.
I used the airflow 2 vacuume fill tool to replace the coolant. Great tool. It also pressure tests the system.