How is an ILS approach conducted when following a heavy aircraft?
Other bodies may advise differently but I will answer for the FAA. 

The FAA covers this in their wake turbulence handbook as well as many other scenarios you can look over.

 

Basically you can fly above the glide slope as you have assumed but you should always take precaution as it cant be known if the leading aircraft is at or above slope. 

As in similar situations, separation services may be provided, load permitting. 


  2.10.5.1 Air Traffic Control Assist 
  
  Air traffic controllers are able to provide sepa- ration  distance 
  information to  pilots  when workload  permits  and  they  have  radar
  dis- plays in the control tower.  They can provide airspeed
  differential between aircraft and may advise pilots following another
  aircraft when they are overtaking the preceding aircraft.


The document outlines some other methods and is worth a read.