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.
Its located here (photo taken after a disconnected and moved the loom plug from the side of the header tank, and disconnected the connector from HCV).
It doesnt take a rocket surgeon to see something is wrong.
I didnt take any photos of removal as its a bit messy, but basically you pull the HCV up and out of its rubber mount, and then remove the hoses. You can then proceed to remove the HCV. Reassembly is the reverse of disassembly.
Here is the replacement in place.
And the reason the heater didnt work?
Its plugged solid. Yum.
Anyway, the heater now works amazingly. Quick to heat up on both sides, and hotter than hell. Great Success.