Things can only get better?

We have a Corsa in for a cooling system overheating problem. Initially it seemed that water was not flowing and the top hose was cold and the bottom hose was hot. It appeared like the thermostat was closed all the time. I changed the thermostat and ran the engine until all hoses were hot, the fan came in and at that time I thought its fixed. The following day the customer returns saying that the coolant has boiled over. I wondered if the pressure cap was defective so replaced with a new one. The next day the same happens. I'm now suspecting the water pump impellor has detached? This is the key point though...

I'm told to ensure that no air is trapped in the system. I'm told to remove the bottom radiator hose and drain the coolant into a bucket and fill the cooling system from the top reservoir while the bottom hose is off to prevent the ingress of air into the cooling system?

I'm done!