Steering rack mounts - £30
Not necessarily the first thing you might have on your upgrade list, these are a direct replacement for the tired rubber rack mounts.
An upgrade even from poly mounts, they add a little more directness and precision to the steering, eliminating any movement of the rack itself. Don't think it's there? Have someone static steer your car whilst watching the rack.
Not really a modification for a road car unless you're hardcore, they will make it a bit more harsh. Legal for ALL classes of XJS or saloon in the JEC series.
Made by a Bear and his lathe, and can be fitted by even the most inept.