2006 Vauxhall Corsa engine overheating

The temperature warning light came on in my car (it doesn't have a temp gauge fitted) when I check the coolant reservoir it was empty I topped it up every day for about a week and booked the car into a garage to be checked out. A few days before it was due to go in the car the warning light came back on again and the car lost power. I called the AA who discovered the connection to the hose on top of the water pump had broke off and coolant was leaking from there.

The connection was replaced but when driving home the warning light came back on after driving about 5 miles. I called the garage and arranged to take it back in the following morning. They have since checked the water pump, the thermostat, hoses etc & did a dye check on the cyclinder head gasket which were all fine but the engine is still overheating and the warning light coming on intermittently anyone had the same problem or ideas whats wrong?