I did change back to the original plugs which made no difference. I swapped around the coils which didn't change anything. As I struggled to fit them in through the tight gap I was wondering if the coil boots might have cracked. I'm in process of obtaining a new coil to swap/pinpoint fault. But if a coil was shorting to the engine block, can I identify that with a multimeter or otherwise? I couldn't see any sparking/arcing.