The grave owner or the person who purchased the memorial is responsible for it's safety and good repair. We insist on the National Association of Memorial Masons fixing method for all new memorials.  Where an existing memorial is replaced e.g. after removal for burial or an additional inscription, it must be replaced in accordance with the National Association of Memorial Masons guidance.  This should ensure the memorial is safe for a considerable period.  
If a headstone becomes loose or dangerous we will try to contact the owner but occasionally we may have to lay flat any memorial that becomes dangerous.

You should consult your mason with regard to any guarantee and also about insuring your memorial.