Vauxhall Vivaro heaters blowing cold air.

My heaters are blowing cold air.water pump, thermostat has been replaced,
And I have removed the heater matrix there is no leaks or blockage as far as the garage concerned. but no improvement. Air is still getting in the system which the garage has bled at least a dozen times. They think there is a problem with the head gasket so we are at a loss as to the problem. They done a pressure test which was normal it don't lose no coolant at all. When the garage has bled the air out they manage to get it to blow hot air but it only lasts for around 5 mins then slowly goes cold. Any ideas ?????