Thanks both - I think that's a job for the weekend then. I was stood there whilst the tech did the job and he was as careful as he could have been, but number 6's hinged clip on the top came off and he had to clip it back on to get the coil pack out. Other than that he seemed pretty careful - the coil packs are pretty easy to break from what I've seen and read.
What will I see if they've been arcing? A burning/melting of the plastic?
As for checking the plugs, I have the right socket, but I guess I'll need a tool of some sort to check the gapping? How do I adjust the gapping though?
As far as the injectors, I have no idea how many were replaced, but they were done when the car was less than a year old, along with the HPFP. I think the first owner of the car was a BMW garage, so they appear to have just done both in one go, to be safe.
With regards to the injector cleaner, do I have to change my oil after doing this, or should the cleaner not result in any deposits in the oil?