This is impacting on the baths and basins upstairs and is caused by and air block between the bathrooms and the tank in the loft, its becoming increasingly frequent and now happens every 3 days on average. easily cleared by forcing hot water from the mixer tap up the cold water pipes untill the tell tail bubbles can be heard in the the tank in the loft. The cold water tank is up to its maximum level, so there is decent pressure. Has anyone got any suggestions as the wife shouts are getting more menacing everytime she cant flush the loo!!