No matter how awkward you might think changing spark plugs are on any given car, I can almost promise you they're harder on a 645. Fusking stupid thing.
Oh yeah, you can do it without dropping the engine, but you need to push the aircon pipes down waaay past the point of comfort on one bank, and remove the positive battery cables on the other. You then need tiny Pygmy hands to get to pull the coil packs off, and half a dozen wobble joints to get to the plugs themselves for removal. Oh sweet Jesus I'm never doing that again!
Not even my car either, the guy who had the blue E46 M3 on the Wales run flogged that and bought an early 645 cab, with the manual 'box as well of course. His has the fancy adaptive ARB things, and they're really good so I've made up my mind to change mine ASAP now.