Right, so the moment i got home today i decided to carry out some work on the M3.
Since i got the car the heater hasnt worked (i knew this when purchased). Neither side changed temp at all, just stuck at cold. I knew this was the heater control valve (HCV), as they are a common failure point as they are easy to block and jam. I purchased some parts from Ray at HellBM, which included a replacement valve.
Its not a hard part to get to on the M3, just a bit tight due to the header tank, and there isnt a lot of space to twist and pull the hoses.