The Decca/London is tricky because its’ lateral and vertical compliances are very different, the lateral being about 50% higher than the vertical. Schizophrenic! Of course, the effective (moving) mass of all arms is partially determined by the distance of the rear counterweight from the arms’ bearings. To increase an arms’ em, move the counterweight further away from the bearings. To keep the tracking force the same, additional mass is then added to the headshell, thereby further increasing the arms’ em. Serendipity! An arm with medium lateral mass and high vertical would be optimum for the Decca/London. Is there such a beast?
A good way to add mass to the "classically"-styled Decca/Londons (the Maroon/Gold/Super Gold, etc.) is to order it with the optional Decapod top plate/mount. A second benefit to the welcome added mass of the pod is the far superior mounting of the cartridge to the arm it affords. The stock cartridge holder/mount is a flimsy joke, completely unacceptable. Owners of stock cartridges can have the Decapod installed at the factory at the same time retipping is done. Well worth its’ modest price.