Hello Stuart,
Yes, you must remove the pulley before all the pump mounting bolts can be removed. A special puller is tool of choice. The 280SL has the tapered pump shaft and pulley. If no puller is available, very gently put some pressure on the pulley (do not bend the pulley) wind the nut out flush to the end of the shaft, and while carefully prying, smack the end of the shaft with a soft mallet (wood or plastic). The pulley should pop loose. The earlier cars (early 230SL) did not have the tappered shaft and removal can be more difficult. Don't loose track of the spacer on the rear mounting bolt!
Joe Alexander
Blacklick, Ohio