Looks like nobody would have anything to worry about when bringing parents over for a short visit considering the following rule from http://www.legislation.gov.uk/ukpga/1985/68/part/X
But UKBA doesn't appear to be referring to this at all!329 Exception; visiting member of family.
Where the persons sleeping in an overcrowded dwelling include a member of the occupier’s family who does not live there but is sleeping there temporarily, the occupier is not guilty of an offence under section 327 (occupier causing or permitting overcrowding) unless the circumstances are such that he would be so guilty if that member of his family were not sleeping there.
