Ok, after I have calmed down, I inspected the engine bay with a torch.
It's not a faulty water pump, but a coolant hose. The coolant hose at the bottom of the engine, leading into the radiator, has come loose. As soon as the thermostat opens, coolant comes gushing out of the open pipe.
Not as serious as a faulty water pump, but still directly as a result of the service department's mechanics