camera flashing in film
whats the technical name of this 'effect'/issue?


The horizontal lines are likely caused by an effect called Rolling Shutter


  why is it more pronounced in slow motion (does interlacing effect this)?
  I would have expected interlacing to reduce the visibility of the effect iff it was done on a line of pixels at a time.  However, if bands of pixels are read during the interlacing, that might make the effect worse.  That would provide a stripe of pixels at a similar exposure level and a step change when the next stripe was read.  
  
  is there anything I can do with regards to shutter/speeds or utilizing 'clear scan' in the c100 to reduce this?


I don't believe so.  It is usually related to lower light levels so anything you can do to get more light into the camera would help.  You might find that reducing the exposure time (faster shutter speed) might help as it would force the camera to complete all the scan in less time.  

(Seems a bit unsporting to use some of the question comments as an answer but they matched and expanded on what I knew.)