The receiver has presumably been appointed by the Court of Protection because of the mental health problems of the flat owner.
I don't think you could officially find out the state of health of the flat owner & am not sure why you want to, because any dealings you have would be with the receiver - who will be responsible for acting on behalf of the owner. Whether it will cause any problems for you depends on how the receiver goes about carrying out their responsibilities but if they do it diligently you should not be any more concerned than if you were dealing direct with the owner.