A maintenance visit will be required , a competent time served plumbing & heating engineer will fire up the heating system & close each radiator apart from 2 on the 1st & ground floor on the system , if its diagnosed at cold spots then on older systems this is where sludge builds up & restricts the flow & return course of the heating through the radiators , air locks are also common once works & alterations have been carried out , automatic air vents on towel warmers are recommended to aid the air escaping from the system as towel warmers are normally the higher points of heating systems on existing installations , a competent plumbing & heating engineer will be able to diagnose & remedy this , if a new system then its more than likely an air lock , the heating will need to be rebalanced after the remedial works have been actioned , hope this helps