Since I've had the car I've been battling a rough idle when cold, and more recently I've noticed an occasional small puff of blue smoke on startup. It did this at seemingly random intervals with no noticeable pattern. After a fair amount of reading online I decided my issue most likely stemmed from the PCV, which is located at the back of the engine. Rather than replacing the entire PCV unit (which never rarely fails), most people opt to replace the rubber diaphragm only which is far easier to remove. These rubber diaphragms are notorious for developing cracks or small holes which enables oil to seep through and thus create a puff of blue smoke on startup.
I decided to try and tackle this job on my own with the aid of several online videos and armed with a flat head screwdriver (only required tool). At first access proved to be an issue, as most of the videos online were done using an E38 which has much more space at the back of the engine. I decided to remove all the plastic panels and other bits which freed up a lot more space. This made the job much easier and I was able to get access to the PCV, pop off the plastic cover and remove the old diaphragm (pictured below - I'll let you decide which one is the old).
The old diaphragm was buggered, and fell to bits as I took it out. I installed the new diaphragm, put the car back together and took it for a test drive (pictured below - clearly much more powerful!). Upon start up it was still pretty rough and there was a small puff of smoke which was slightly disheartening, but most people say the system takes a few days to clear its throat. Once on the road the power deliver felt noticeably smoother and the engine overall happier. Over the next week or so there were probably 3-4 very small puffs of smoke on startup, which was completely eradicated after a week of driving. What's more, the cold idle was 100x smoother. Still slightly lumpy but a huge improvement.
This is the most involved job I've ever done on a car by myself, which was satisfying and rewarding when it actually worked. I fixed something! This car is a great base to learn to wrench on cars in my opinion. It's great getting below the surface and discovering all of the other design elements of the car that 90% of owners will never see. Removing all of the plastic panels blocking access to the PCV was so methodical and beautifully simple (I'm sure not everything is this way!).