I did a full flush just the other day. As mentioned above, you'll need to drain the radiator, and the block itself. A picture of the block drain on the M103 is attached below. I think it's 17mm and with about a 6 inch socket extention you can reach it from above without going underneath the car to turn it.
Drain all coolant then flush with appropriate agent (citric for buildup; degreaser for any signs of oil -- SHOUT! works well, 3 bucks at your local grocer)
After the flush, fill with regular water and run engine some more. Drain again. Now take a water hose with both drain plugs open and stick it in resevoir filler until you see clear water draining from both locations. Tighten drain plugs and fill'er up with coolant/water and you're good to go.
FWIW you'll find that your auto climate control works better after a good flush. Don't ask me why but I actually get fan speed changes now when in auto mode, where before I didn't. Maybe someone else can explain this one.
Looking at left side of engine: