Some owners report a workaround: a fabricated special tool and a documented procedure allow the spark plugs to be changed without removing the engine, at far lower cost. This is described as a non-factory-approved method available through independent specialists rather than a franchised dealer, and it coexists with the official engine-out job rather than replacing it.
Changing plugs without pulling the engine
Mercedes-Benz SLR McLaren (C199) · ownership