Hot water is allowing air in - somewhere, so the hot tap is spluttering until water flows. non return valve has been replaced, and the air cushion action has been performed. but still air is getting in somewhere . any suggestions, ?
I think you possibly need an engineer to check this as it sounds as though the work as not been done correctly or the parts are faulty. The one thing that comes to mind assuming this is a drop in pump is that it should be dropped into the water before it is turned on.