Neil, I appreciate that you investigated the rattle issue to some extent, but did you not take the car back to the dealer and ask them to get it resolved under the warranty, I presume it was still under the 2 years warranty at the time and this would have covered any engine issues?
Yes I had many conversations with the dealer - and other dealers too. Unfortunately it wasn't resolved... gently dropped as a topic I'd suggest. It's worth adding that because this is a relatively new model, the engines are a little unknown to the dealers. Bit of a sealed unit in some respects. Is an engine rattle a warranty claim? Is it a 'feature' of the engine? It's hard to get traction to be honest.