I also watched it.
The theory about the French cyclist being the actual target and the family being shot for being accidental witnesses doesn't really wash if the murderer was rendered unrecogniseable by wearing motorcycle gear, don't you think?
The back-story to the cyclist is fascinating and enough to throw an interesting level of doubt over the whole case. What are the odds of the accidental witness/extra victim being inveigled with both a nuke plant and a rich family who didn't like his relationship with their daughter? Totally wierd!
That thing about the stream helpfully masking the sound of gunfire - suggests the perpetrator had a great deal of local knowledge... but could, equally just be down to luck. A local would also be aware of the English cyclist riding the track on a regular basis, you'd have thought.
Plenty of food for thought. Where's me deerstalker?