If your Disco has steering wander, jack up the front wheels and check for wheel play.
Mine (2000 Disco td5) has been getting worse over the past year, and a recent check revealed excessive play in two planes from the front drivers side.
In my case it was coming from worn hub bearings. There was no bearing rumble, just play. Today I fitted a new hub assembly, and she is tracking "straight as a die!"
Very noticeable improvement in driver comfort (and safety) after such a long period of wander, I guess we just slowly compensate over time and don't notice the wander as much as we should.

Hardest part was the hub nut.
With no access to a 3/4 drive long six-sided socket and lengthy breaker-bar, I ended up welding about 150mm length of approx 30mm diameter water pipe onto a 32mm standard socket. The benefit was almost 1 to 1 drive, enabled by a large plumbers wrench, and a 2 metre length of pipe for extra "purchase." I also placed a jack at the distal end of my home-made socket extension to prevent slippage.
It worked, but things are much easier if you have access to correct tools!