They are two separate offences.
#1 pervert the course of justice
pervert: alter something from its original/intended course
In law, there are three forms of this :
1. when someone fabricates evidence, or the disposal of evidence
2. if someone intimidates a witness or juror by subjecting them to fear
3. someone threatens a witness or juror with physical harm, (to self or family)
Obstruct the course of justice:
This is hindering the course of justice, as in hiding a criminal, or withholding important information that would help police or the courts.