Hi, it's been a while since I've been able to work on the car due to family issues, but before I left I realised it was no.6 injector leaking at the return line. To understand if the smoking was possible to come from this diesel fuel I tipped some water behind the valve cover (the exact place where diesel would spill if enough accumulated) and sure enough, with the engine being on a slight angle the water dropped straight onto the DPF and surrounding area. So, this would explain why the smoke was so random, it was just a matter of time before it accumulated enough and with a steep hill or speed bump the diesel would eventually spill down the back.