HI,
Just had the same problem as you had. i changed the head gasket anyway, took a while but didn't look like the gasket had gone. when i came to refill the coolant, found that the heater matrix wasn't filling. therefore air-lock, tried filling by the book method but still wouldn't fill, so put hose pipe into the matrix itself and quickly replaced hose onto the head. seems to have sorted problem.
regards guy