Thanks for the replies. I'll assume that the strange contamination on that plug is due to a bit of oil running down from the filler cap, especially as the electrode seems normal (I think). As for the plugs being original, well the car has no history with it at all and has been a bit neglected in the past, so I'm doing my best to get it back to a decent state!
In case anyone is looking for some info on this, it took me an hour and a half to do all six and required little more than a basic socket set with 5" and 10" extension bars. I also have a 'mini' socket set for removing the smaller bolts. I removed the rubber bung from the spark plug socket as I have an extending magnet thingy for pulling them - also comes in useful when you drop your sockets/bolts/screws deep into the engine bay!
Oh and this is the first time I've replaced spark plugs on any car so I'm not experienced at this at all!
Here are a few pics that I've tried to annotate a little;
This shows the three panels that I removed after removal of the acoustic engine cover - so four bits to remove in order to gain enough access.
This shows the location of the seven bolts that need to be removed from the coil packs. (Three on one and two on the others).
And this is what the electrodes look like. Pretty normal as far as i can tell.
